LU-15278 mdt: protect layout xattr

LU-15278 mdt: protect layout xattr

During layout change, the layout xattr could be simultaneously
used by others.

This patch put the server LOVEA access in the protection of
mdt_object::mot_lov_mutex.

Lustre-change: https://review.whamcloud.com/45663
Lustre-commit: TBD (from 48bb7206fb656a35283ea7eefdec208dd6c42078)

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Iabad4a3211b7ef74592d92b86082201687964ab2
Reviewed-on: https://review.whamcloud.com/46262
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46327