xfs: fix incorrect context handling in xfs_trans_roll

The memalloc_nofs_save() and memalloc_nofs_restore() calls are
incorrectly paired in xfs_trans_roll.

Call path:
xfs_trans_alloc()
    __xfs_trans_alloc()
	// tp->t_pflags = memalloc_nofs_save();
	xfs_trans_set_context()
...
xfs_defer_trans_roll()
    xfs_trans_roll()
        xfs_trans_dup()
            // old_tp->t_pflags = 0;
            xfs_trans_switch_context()
        __xfs_trans_commit()
            xfs_trans_free()
                // memalloc_nofs_restore(tp->t_pflags);
                xfs_trans_clear_context()

The code passes 0 to memalloc_nofs_restore() when committing the original
transaction, but memalloc_nofs_restore() should always receive the
flags returned from the paired memalloc_nofs_save() call.

Before commit 3f6d5e6 ("mm: introduce memalloc_flags_{save,restore}"),
calling memalloc_nofs_restore(0) would unset the PF_MEMALLOC_NOFS flag,
which could cause memory allocation deadlocks[1].
Fortunately, after that commit, memalloc_nofs_restore(0) does nothing,
so this issue is currently harmless.

Fixes: 756b1c3 ("xfs: use current->journal_info for detecting transaction recursion")
Link: https://lore.kernel.org/linux-xfs/20251104131857.1587584-1-leo.lilong@huawei.com [1]
Signed-off-by: Wenwu Hou <hwenwur@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: hwenwur

fs/xfs/xfs_trans.c

@@ -124,8 +124,6 @@ xfs_trans_dup(
 	ntp->t_rtx_res = tp->t_rtx_res - tp->t_rtx_res_used;
 	tp->t_rtx_res = tp->t_rtx_res_used;
 
-	xfs_trans_switch_context(tp, ntp);
-
 	/* move deferred ops over to the new tp */
 	xfs_defer_move(ntp, tp);
 
@@ -1043,6 +1041,12 @@ xfs_trans_roll(
 	 * locked be logged in the prior and the next transactions.
 	 */
 	tp = *tpp;
+	/*
+	 * __xfs_trans_commit cleared the NOFS flag by calling into
+	 * xfs_trans_free.  Set it again here before doing memory
+	 * allocations.
+	 */
+	xfs_trans_set_context(tp);
 	error = xfs_log_regrant(tp->t_mountp, tp->t_ticket);
 	if (error)
 		return error;

fs/xfs/xfs_trans.h

@@ -280,13 +280,4 @@ xfs_trans_clear_context(
 	memalloc_nofs_restore(tp->t_pflags);
 }
 
-static inline void
-xfs_trans_switch_context(
-	struct xfs_trans	*old_tp,
-	struct xfs_trans	*new_tp)
-{
-	new_tp->t_pflags = old_tp->t_pflags;
-	old_tp->t_pflags = 0;
-}
-
 #endif	/* __XFS_TRANS_H__ */