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>