LU-10321 lfsck: not start lfsck during umount

LU-10321 lfsck: not start lfsck during umount

There is race condition bewtween lfsck_start and umount:
the LFSCK may be triggered just after the LFSCK stopped
during umount the target, then nobody will stop the new
started LFSCK, as to the umount may be blocked.

This patch sets flag on the lfsck instance when umount
that will prevent subsequent lfsck_start.

This is a port to b2_10 of
Lustre-change: https://review.whamcloud.com/#/c/30513/
Lustre-commit: 6263064b369dd4fddbb0dfa9ab49013a0d791629

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I779f862d4195d4289bb9dd96575cd7746ac4b35b
Reviewed-on: https://review.whamcloud.com/30514
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>