LU-2219 ptlrpc: so_hpreq_handler is set twice for the ost_io svc

LU-2219 ptlrpc: so_hpreq_handler is set twice for the ost_io svc

ptlrpc_service_conf.psc_ops.so_hpreq_handler is set twice for
the ost_io service in ost_setup(); the second assignment
overwrites the first to NULL, so ost_io threads would never
handle RPCs as high-priority ones.

While we are at it, remove some superfluous assignments of
so_hpreq_handler to NULL for statically allocated
ptlrpc_service_conf structs when initializing other ptlrpc
services, and rename some relevant functions.

Signed-off-by: Nikitas Angelinas <nikitas_angelinas@xyratex.com>
Change-Id: Ia728a3d7f20511fcb58b259126b05055d5860455
Xyratex-bug-id: MRP-724
Reviewed-on: http://review.whamcloud.com/4368
Tested-by: Hudson
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>