xfs: restore old agirotor behavior

Prior to the removal of xfs_ialloc_next_ag, we would increment the agi
rotor and return the *old* value.  atomic_inc_return returns the new
value, which causes mkfs to allocate the root directory in AG 1.  Put
back the old behavior (at least for mkfs) by subtracting 1 here.

Fixes: 20a5eab4 ("xfs: convert xfs_ialloc_next_ag() to an atomic")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/libxfs/xfs_ialloc.c

@@ -1729,7 +1729,8 @@ xfs_dialloc(
 	 * an AG has enough space for file creation.
 	 */
 	if (S_ISDIR(mode))
-		start_agno = atomic_inc_return(&mp->m_agirotor) % mp->m_maxagi;
+		start_agno = (atomic_inc_return(&mp->m_agirotor) - 1) %
+				mp->m_maxagi;
 	else {
 		start_agno = XFS_INO_TO_AGNO(mp, parent);
 		if (start_agno >= mp->m_maxagi)