LU-933 mdc: allow disabling the mdc_rpc_lock

LU-933 mdc: allow disabling the mdc_rpc_lock

It is desirable to allow disabling the client mdc_{get,put}_rpc_lock()
in order to allow clients to send multiple filesystem-modifying RPCs
at the same time. While this would break MDS recovery (due to
insufficient transaction slots in the MDS last_rcvd file) it would
allow a smaller number of clients to generate a much higher RPC load
on the MDS. This is ideal for MDS/RPC load testing purposes, and can
also be used to help evaluate the potential benefits of implementing
the multi-slot last_rcvd feature.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: I39be390ed37f0327bf7eb8015d528195b8568e0b
Reviewed-on: http://review.whamcloud.com/2084
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>