nfs/localio: restore creds before releasing pageio data

Otherwise if the nfsd filecache code releases the nfsd_file
immediately, it can trigger the BUG_ON(cred == current->cred) in
__put_cred() when it puts the nfsd_file->nf_file->f-cred.

Fixes: b9f5dd5 ("nfs/localio: use dedicated workqueues for filesystem read and write")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Link: https://lore.kernel.org/r/20250807164938.2395136-1-smayhew@redhat.com
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

Author: scottmayhew

fs/nfs/localio.c

@@ -453,12 +453,13 @@ static void nfs_local_call_read(struct work_struct *work)
 	nfs_local_iter_init(&iter, iocb, READ);
 
 	status = filp->f_op->read_iter(&iocb->kiocb, &iter);
+
+	revert_creds(save_cred);
+
 	if (status != -EIOCBQUEUED) {
 		nfs_local_read_done(iocb, status);
 		nfs_local_pgio_release(iocb);
 	}
-
-	revert_creds(save_cred);
 }
 
 static int
@@ -648,14 +649,15 @@ static void nfs_local_call_write(struct work_struct *work)
 	file_start_write(filp);
 	status = filp->f_op->write_iter(&iocb->kiocb, &iter);
 	file_end_write(filp);
+
+	revert_creds(save_cred);
+	current->flags = old_flags;
+
 	if (status != -EIOCBQUEUED) {
 		nfs_local_write_done(iocb, status);
 		nfs_local_vfs_getattr(iocb);
 		nfs_local_pgio_release(iocb);
 	}
-
-	revert_creds(save_cred);
-	current->flags = old_flags;
 }
 
 static int