xfs: fix frozen file system assert in xfs_trans_alloc

commit 647b3d5 upstream.

Commit 83a80e9 ("xfs: decouple xfs_trans_alloc_empty from
xfs_trans_alloc") move the place of the assert for a frozen file system
after the sb_start_intwrite call that ensures it doesn't run on frozen
file systems, and thus allows to incorrect trigger it.

Fix that by moving it back to where it belongs.

Fixes: 83a80e9 ("xfs: decouple xfs_trans_alloc_empty from xfs_trans_alloc")
Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Christoph Hellwig

fs/xfs/xfs_trans.c

@@ -284,8 +284,8 @@ xfs_trans_alloc(
 	 * by doing GFP_KERNEL allocations inside sb_start_intwrite().
 	 */
 retry:
-	WARN_ON(mp->m_super->s_writers.frozen == SB_FREEZE_COMPLETE);
 	tp = __xfs_trans_alloc(mp, flags);
+	WARN_ON(mp->m_super->s_writers.frozen == SB_FREEZE_COMPLETE);
 	error = xfs_trans_reserve(tp, resp, blocks, rtextents);
 	if (error == -ENOSPC && want_retry) {
 		xfs_trans_cancel(tp);