xfs: count the blocks in a btree

Provide a helper method to count the number of blocks in a short form
btree.  The refcount and rmap btrees need to know the number of blocks
already in use to set up their per-AG block reservations during mount.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
index 50cfb98..aa1752f 100644
--- a/fs/xfs/libxfs/xfs_btree.c
+++ b/fs/xfs/libxfs/xfs_btree.c
@@ -4821,3 +4821,26 @@
 	}
 	return rval;
 }
+
+int
+xfs_btree_count_blocks_helper(
+	struct xfs_btree_cur	*cur,
+	int			level,
+	void			*data)
+{
+	xfs_extlen_t		*blocks = data;
+	(*blocks)++;
+
+	return 0;
+}
+
+/* Count the blocks in a btree and return the result in *blocks. */
+int
+xfs_btree_count_blocks(
+	struct xfs_btree_cur	*cur,
+	xfs_extlen_t		*blocks)
+{
+	*blocks = 0;
+	return xfs_btree_visit_blocks(cur, xfs_btree_count_blocks_helper,
+			blocks);
+}