LU-16973 osd: adds SB_KERNMOUNT flag

LU-16973 osd: adds SB_KERNMOUNT flag

During umount mntput() is called. It uses delayed_mntput()
function, and it could take much time to finish. A block
device is occupied during delayed work.

[ 8753.941980] Lustre: server umount XXX complete
[ 8800.129136] sysrq: SysRq : Trigger a crash

PID: 319306   TASK:XXXX   CPU: 2    COMMAND: "kworker/2:0"
 #0 __schedule at ffffffff9754e1d4
 #1 preempt_schedule_common at ffffffff9754e6fa
 #2 _cond_resched at ffffffff9754e72d
 #3 invalidate_mapping_pages at ffffffff96e72da5
 #4 invalidate_bdev at ffffffff96f5d13c
 #5 ldiskfs_put_super at ffffffffc1c82e34 [ldiskfs]
 #6 generic_shutdown_super at ffffffff96f1bdcc
 #7 kill_block_super at ffffffff96f1bed1
 #8 deactivate_locked_super at ffffffff96f1b784
 #9 cleanup_mnt at ffffffff96f3b86b

Let's use SB_KERNMOUNT flag during mount, it leads to
synchronous mntput().
It also calls flush_delayed_fput during umount to finish
delayed fput.

Lustre-change: https://review.whamcloud.com//51731
Lustre-commit: eff11c8ce1f89f30dcc5af88b67b3d6c15a631a6

Change-Id: Ia6729f6cbac85c3626562e946a4b96665a143714
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52495
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>