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.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I437c919e498781b8d5dc653bf90aac799df4882a
Reviewed-on: https://review.whamcloud.com/43366
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>