LU-9615 mdt: use write llog lock for pending restores

LU-9615 mdt: use write llog lock for pending restores

In mdt_hsm_pending_restore(), ask for a write llog lock when
processing the llog.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ib93a3a3b0f9472afae50c240b7dcb14fce4a8501
Reviewed-on: https://review.whamcloud.com/27484
Tested-by: Jenkins
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>