LU-1717 ldlm: Fix rs_opc initialization

LU-1717 ldlm: Fix rs_opc initialization

By the time target_send_reply() initializes rs_opc, rs_msg has not been
filled with a valid opc yet.  Following Oleg's suggestion on the Jira
ticket, this patch changes target_send_reply() to initialize rs_opc with
rq_reqmsg instead and silences a couple of related warnings that are of
only informative nature.

Change-Id: I4b96454e0bcf3dd0dc8f21b0de70a89ce37faacf
Signed-off-by: Li Wei <liwei@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4271
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>