gfs2: Add metapath_dibh helper

Add a metapath_dibh() helper for extracting the inode's buffer head from
a metapath.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/bmap.c

@@ -317,6 +317,12 @@ static void gfs2_metapath_ra(struct gfs2_glock *gl, __be64 *start, __be64 *end)
 	}
 }
 
+static inline struct buffer_head *
+metapath_dibh(struct metapath *mp)
+{
+	return mp->mp_bh[0];
+}
+
 static int __fillup_metapath(struct gfs2_inode *ip, struct metapath *mp,
 			     unsigned int x, unsigned int h)
 {
@@ -659,7 +665,7 @@ static int __gfs2_iomap_alloc(struct inode *inode, struct iomap *iomap,
 {
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_sbd *sdp = GFS2_SB(inode);
-	struct buffer_head *dibh = mp->mp_bh[0];
+	struct buffer_head *dibh = metapath_dibh(mp);
 	u64 bn;
 	unsigned n, i, blks, alloced = 0, iblks = 0, branch_start = 0;
 	size_t dblks = iomap->length >> inode->i_blkbits;