LU-5312 ptlrpc: update export_last_request_time after recovery

Update export_last_request_time after recovery, because client can
not send ping to the server during the recovery, so we need refresh
the last_request_time to avoid the export is being evicted earlier
than it should be.

Change-Id: I1d9bf5aaa3ca4215479ddc942c7052ad047c6989
Signed-off-by: Wang Di <di.wang@intel.com>
Reviewed-on: http://review.whamcloud.com/11443
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>