LU-3172 ost: call lustre_msg_get_opc() once inside ost_handle()

lustre_msg_get_opc() is called a few times inside ost_handle();
there is no real benefit from this, and we can just make one call
when entering the function.

There may be other occurrences in the source where this applies,
apart from ost_handle(), but we can at least make this change here
for now, as this is a frequently-called function.

Signed-off-by: Nikitas Angelinas <nikitas_angelinas@xyratex.com>
Change-Id: I7a8badc30ca31cb6826463ae5390cef96dec345f
Xyratex-bug-id: MRP-698
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Reviewed-on: http://review.whamcloud.com/6055
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>