nfs/localio: fix regression due to out-of-order __put_cred

Commit f2060bd ("nfs/localio: add refcounting for each iocb IO
associated with NFS pgio header") inadvertantly reintroduced the same
potential for __put_cred() triggering BUG_ON(cred == current->cred) that
commit 992203a ("nfs/localio: restore creds before releasing pageio
data") fixed.

Fix this by saving and restoring the cred around each {read,write}_iter
call within the respective for loop of nfs_local_call_{read,write} using
scoped_with_creds().

NOTE: this fix started by first reverting the following commits:

 94afb62 ("nfs: use credential guards in nfs_local_call_read()")
 bff3c84 ("nfs: use credential guards in nfs_local_call_write()")
 1d18101 ("Merge tag 'kernel-6.19-rc1.cred' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs")

followed by narrowly fixing the cred lifetime issue by using
scoped_with_creds().  In doing so, this commit's changes appear more
extensive than they really are (as evidenced by comparing to v6.18's
fs/nfs/localio.c).

Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Acked-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/linux-next/20251205111942.4150b06f@canb.auug.org.au/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Mike Snitzer

fs/nfs/localio.c

@@ -615,8 +615,11 @@ static void nfs_local_read_aio_complete(struct kiocb *kiocb, long ret)
 	nfs_local_pgio_aio_complete(iocb); /* Calls nfs_local_read_aio_complete_work */
 }
 
-static void do_nfs_local_call_read(struct nfs_local_kiocb *iocb, struct file *filp)
+static void nfs_local_call_read(struct work_struct *work)
 {
+	struct nfs_local_kiocb *iocb =
+		container_of(work, struct nfs_local_kiocb, work);
+	struct file *filp = iocb->kiocb.ki_filp;
 	bool force_done = false;
 	ssize_t status;
 	int n_iters;
@@ -633,7 +636,9 @@ static void do_nfs_local_call_read(struct nfs_local_kiocb *iocb, struct file *fi
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
-		status = filp->f_op->read_iter(&iocb->kiocb, &iocb->iters[i]);
+		scoped_with_creds(filp->f_cred)
+			status = filp->f_op->read_iter(&iocb->kiocb, &iocb->iters[i]);
+
 		if (status != -EIOCBQUEUED) {
 			if (unlikely(status >= 0 && status < iocb->iters[i].count))
 				force_done = true; /* Partial read */
@@ -645,16 +650,6 @@ static void do_nfs_local_call_read(struct nfs_local_kiocb *iocb, struct file *fi
 	}
 }
 
-static void nfs_local_call_read(struct work_struct *work)
-{
-	struct nfs_local_kiocb *iocb =
-		container_of(work, struct nfs_local_kiocb, work);
-	struct file *filp = iocb->kiocb.ki_filp;
-
-	scoped_with_creds(filp->f_cred)
-		do_nfs_local_call_read(iocb, filp);
-}
-
 static int
 nfs_local_do_read(struct nfs_local_kiocb *iocb,
 		  const struct rpc_call_ops *call_ops)
@@ -822,13 +817,18 @@ static void nfs_local_write_aio_complete(struct kiocb *kiocb, long ret)
 	nfs_local_pgio_aio_complete(iocb); /* Calls nfs_local_write_aio_complete_work */
 }
 
-static ssize_t do_nfs_local_call_write(struct nfs_local_kiocb *iocb,
-				       struct file *filp)
+static void nfs_local_call_write(struct work_struct *work)
 {
+	struct nfs_local_kiocb *iocb =
+		container_of(work, struct nfs_local_kiocb, work);
+	struct file *filp = iocb->kiocb.ki_filp;
+	unsigned long old_flags = current->flags;
 	bool force_done = false;
 	ssize_t status;
 	int n_iters;
 
+	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
+
 	file_start_write(filp);
 	n_iters = atomic_read(&iocb->n_iters);
 	for (int i = 0; i < n_iters ; i++) {
@@ -842,7 +842,9 @@ static ssize_t do_nfs_local_call_write(struct nfs_local_kiocb *iocb,
 		} else
 			iocb->kiocb.ki_flags &= ~IOCB_DIRECT;
 
-		status = filp->f_op->write_iter(&iocb->kiocb, &iocb->iters[i]);
+		scoped_with_creds(filp->f_cred)
+			status = filp->f_op->write_iter(&iocb->kiocb, &iocb->iters[i]);
+
 		if (status != -EIOCBQUEUED) {
 			if (unlikely(status >= 0 && status < iocb->iters[i].count))
 				force_done = true; /* Partial write */
@@ -854,22 +856,6 @@ static ssize_t do_nfs_local_call_write(struct nfs_local_kiocb *iocb,
 	}
 	file_end_write(filp);
 
-	return status;
-}
-
-static void nfs_local_call_write(struct work_struct *work)
-{
-	struct nfs_local_kiocb *iocb =
-		container_of(work, struct nfs_local_kiocb, work);
-	struct file *filp = iocb->kiocb.ki_filp;
-	unsigned long old_flags = current->flags;
-	ssize_t status;
-
-	current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
-
-	scoped_with_creds(filp->f_cred)
-		status = do_nfs_local_call_write(iocb, filp);
-
 	current->flags = old_flags;
 }