gfs2: Fix iomap buffered write support for journaled files

Commit 64bc06bb broke buffered writes to journaled files (chattr
+j): we'll try to journal the buffer heads of the page being written to
in gfs2_iomap_journaled_page_done.  However, the iomap code no longer
creates buffer heads, so we'll BUG() in gfs2_page_add_databufs.  Fix
that by creating buffer heads ourself when needed.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/bmap.c

@@ -975,6 +975,10 @@ static void gfs2_iomap_journaled_page_done(struct inode *inode, loff_t pos,
 {
 	struct gfs2_inode *ip = GFS2_I(inode);
 
+	if (!page_has_buffers(page)) {
+		create_empty_buffers(page, inode->i_sb->s_blocksize,
+				     (1 << BH_Dirty)|(1 << BH_Uptodate));
+	}
 	gfs2_page_add_databufs(ip, page, offset_in_page(pos), copied);
 }