staging/lustre/ptlrpc: add rpc_cache

Add rpc_cache for allocating ptlrpc_requests.

Xyratex-bug-id: MRP-689
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-on: http://review.whamcloud.com/6874
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2424


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>