ocfs2: use the OCFS2_XATTR_ROOT_SIZE macro in ocfs2_reflink_xattr_header()

Use the OCFS2_XATTR_ROOT_SIZE macro improves the readability of the
code.

Link: http://lkml.kernel.org/r/5A2E2488.70301@huawei.com
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Reviewed-by: Jun Piao <piaojun@huawei.com>
Cc: Mark Fasheh <mfasheh@versity.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Joseph Qi <jiangqi903@gmail.com>
Cc: Changwei Ge <ge.changwei@h3c.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ocfs2/xattr.c

@@ -6415,7 +6415,7 @@ static int ocfs2_reflink_xattr_header(handle_t *handle,
 		 * and then insert the extents one by one.
 		 */
 		if (xv->xr_list.l_tree_depth) {
-			memcpy(new_xv, &def_xv, sizeof(def_xv));
+			memcpy(new_xv, &def_xv, OCFS2_XATTR_ROOT_SIZE);
 			vb->vb_xv = new_xv;
 			vb->vb_bh = value_bh;
 			ocfs2_init_xattr_value_extent_tree(&data_et,