LU-7038 obdclass: lu_site_purge() to handle purge-all

LU-7038 obdclass: lu_site_purge() to handle purge-all

if the callers wants to purge all objects, then scanning
should start from the first bucket.

Change-Id: I9f75ff0bd10e1d501e7c790b03ef6a73819a96d1
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/18505
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>