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>