xfs: use the finobt block counts to speed up mount times

Now that we have reliable finobt block counts, use them to speed up the
per-AG block reservation calculations at mount time.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/libxfs/xfs_ialloc_btree.c

@@ -694,6 +694,28 @@ xfs_inobt_count_blocks(
 	return error;
 }
 
+/* Read finobt block count from AGI header. */
+static int
+xfs_finobt_read_blocks(
+	struct xfs_mount	*mp,
+	struct xfs_trans	*tp,
+	xfs_agnumber_t		agno,
+	xfs_extlen_t		*tree_blocks)
+{
+	struct xfs_buf		*agbp;
+	struct xfs_agi		*agi;
+	int			error;
+
+	error = xfs_ialloc_read_agi(mp, tp, agno, &agbp);
+	if (error)
+		return error;
+
+	agi = agbp->b_addr;
+	*tree_blocks = be32_to_cpu(agi->agi_fblocks);
+	xfs_trans_brelse(tp, agbp);
+	return 0;
+}
+
 /*
  * Figure out how many blocks to reserve and how many are used by this btree.
  */
@@ -711,7 +733,11 @@ xfs_finobt_calc_reserves(
 	if (!xfs_sb_version_hasfinobt(&mp->m_sb))
 		return 0;
 
-	error = xfs_inobt_count_blocks(mp, tp, agno, XFS_BTNUM_FINO, &tree_len);
+	if (xfs_sb_version_hasinobtcounts(&mp->m_sb))
+		error = xfs_finobt_read_blocks(mp, tp, agno, &tree_len);
+	else
+		error = xfs_inobt_count_blocks(mp, tp, agno, XFS_BTNUM_FINO,
+				&tree_len);
 	if (error)
 		return error;