LU-17413 llite: protect check in ll_merge_md_attr()

LU-17413 llite: protect check in ll_merge_md_attr()

striping can apply in a concurrent process, so the check for striping
should be serialized against any concurrent process.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Iffac2f1f9b53abc26705d70a30c2201b48156ac8
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53639
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>