xfs: perag initialization should only touch m_ag_max_usable for AG 0

xfs: perag initialization should only touch m_ag_max_usable for AG 0

Source kernel commit: 9789dd9e1d939232e8ff4c50ef8e75aa6781b3fb

We call __xfs_ag_resv_init to make a per-AG reservation for each AG.
This makes the reservation per-AG, not per-filesystem.  Therefore, it
is incorrect to adjust m_ag_max_usable for each AG.  Adjust it only
when we're reserving AG 0's blocks so that we only do it once per fs.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>