staging: lustre: obdclass: lu_site_purge() to handle purge-all

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

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7038
Reviewed-on: http://review.whamcloud.com/18505


Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>