LU-14621 mdd: fix lock-tx order in mdd_xattr_merge()

LU-14621 mdd: fix lock-tx order in mdd_xattr_merge()

to follow common transaction-first-then-locks rule.

Lustre-change: https://review.whamcloud.com/43366
Lustre-commit: b7bd4e3422935fec82d13348d90ec205ac2f4da4

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I437c919e498781b8d5dc653bf90aac799df4882a
Reviewed-on: https://review.whamcloud.com/46235
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>