LU-12949 obdclass: don't extend timer if obd stops

LU-12949 obdclass: don't extend timer if obd stops

During umount all clients became stale, so the first check at
check_for_recovery_ready() is passed, but there is no guarantee
that recovery timer was started. So, we need to check obd_stopping.

The test 138 is added to recovery-smal.sh.
It reproduces the issue when MDT is waiting for clients during
recovery and MDT umount happens.
extend_recovery_timer()) ASSERTION( obd->obd_recovery_start != 0 )
failed

Cray-bug-id: LUS-7917
Signed-off-by: Alexander Boyko <c17825@cray.com>
Change-Id: I1906fdfcc10606912a1f81560bb60b9d424db149
Reviewed-on: https://review.whamcloud.com/36703
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Reviewed-by: Sergey Cheremencev <c17829@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>