LU-11519 hsm: handle hsd_request_count == 0 properly

LU-11519 hsm: handle hsd_request_count == 0 properly

In mdt_cdt_waiting_cb() it may be that the coordinator has already
reached the limit of active requests and hsd contains no requests to
be started. Handle this properly when trying to prioritize a restore.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Ic843b7672ae6a4509ac127c2d2f90bf3681f84fc
Reviewed-on: https://review.whamcloud.com/33580
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Ben Evans <bevans@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>