xfs: perag initialization should only touch m_ag_max_usable for AG 0

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>

fs/xfs/libxfs/xfs_ag_resv.c

@@ -156,7 +156,8 @@ __xfs_ag_resv_free(
 	trace_xfs_ag_resv_free(pag, type, 0);
 
 	resv = xfs_perag_resv(pag, type);
-	pag->pag_mount->m_ag_max_usable += resv->ar_asked;
+	if (pag->pag_agno == 0)
+		pag->pag_mount->m_ag_max_usable += resv->ar_asked;
 	/*
 	 * AGFL blocks are always considered "free", so whatever
 	 * was reserved at mount time must be given back at umount.
@@ -216,7 +217,14 @@ __xfs_ag_resv_init(
 		return error;
 	}
 
-	mp->m_ag_max_usable -= ask;
+	/*
+	 * Reduce the maximum per-AG allocation length by however much we're
+	 * trying to reserve for an AG.  Since this is a filesystem-wide
+	 * counter, we only make the adjustment for AG 0.  This assumes that
+	 * there aren't any AGs hungrier for per-AG reservation than AG 0.
+	 */
+	if (pag->pag_agno == 0)
+		mp->m_ag_max_usable -= ask;
 
 	resv = xfs_perag_resv(pag, type);
 	resv->ar_asked = ask;