LU-17202 target: set SB_KERNMOUNT

LU-17202 target: set SB_KERNMOUNT

set SB_KERNMOUNT on server's mountpoint so umount process block
until the server (MDS/OST/MGS) is really umounted and released
the corresponding block device.

Lustre-change: https://review.whamcloud.com/52724
Lustre-commit: TBD (from 005a85f1c17a6c3477786a47dccfb67f2b36cb09)

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I03ac2dc6bb7cd4d93f5a1729fccb976aa1ebd5d4
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52725
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>