ocfs2: fix bad pointer cast

generic/188 triggered a dmesg stack trace because the dio completion
was casting a buffer head to an on-disk inode, which is whacky.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>

fs/ocfs2/aops.c

@@ -2317,7 +2317,7 @@ static int ocfs2_dio_end_io_write(struct inode *inode,
 			mlog_errno(ret);
 	}
 
-	di = (struct ocfs2_dinode *)di_bh;
+	di = (struct ocfs2_dinode *)di_bh->b_data;
 
 	ocfs2_init_dinode_extent_tree(&et, INODE_CACHE(inode), di_bh);