xfs: add struct xfs_mount pointer to struct xfs_buf

We need to derive the mount pointer from a buffer in a lot of place.
Add a direct pointer to short cut the pointer chasing.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
diff --git a/fs/xfs/xfs_attr_inactive.c b/fs/xfs/xfs_attr_inactive.c
index 228821b..d4f4c96 100644
--- a/fs/xfs/xfs_attr_inactive.c
+++ b/fs/xfs/xfs_attr_inactive.c
@@ -121,7 +121,7 @@ xfs_attr3_leaf_inactive(
 	int			size;
 	int			tmp;
 	int			i;
-	struct xfs_mount	*mp = bp->b_target->bt_mount;
+	struct xfs_mount	*mp = bp->b_mount;
 
 	leaf = bp->b_addr;
 	xfs_attr3_leaf_hdr_from_disk(mp->m_attr_geo, &ichdr, leaf);