LU-8178 lproc: fix negative recovery_duration

LU-8178 lproc: fix negative recovery_duration

There is a window in target_recovery_thread where
obd_recovery_end is not set despite of
obd_recovering becomes 0. In such case
recovery_duration shows difference between 0 and
obd_recovery_start like below:
recovery_duration: 18446744072323902150.

Change-Id: Id385156585c99d12726e89980b55c588e0bf3ceb
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@seagate.com>
Seagate-bug-id: MRP-1519
Reviewed-by: Alexander Nikolaevich Boyko <alexander.boyko@seagate.com>
Reviewed-by: Artem Blagodarenko <artem.blagodarenko@seagate.com>
Tested-by: Parinay Vijayprakash Kondekar <parinay.kondekar@seagate.com>
Reviewed-on: http://review.whamcloud.com/20381
Tested-by: Jenkins
Reviewed-by: Alexander Boyko <alexander.boyko@seagate.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>