LU-1820 ptlrpc: fix req reference to export

LU-1820 ptlrpc: fix req reference to export

(genops.c:766:class_export_destroy())
ASSERTION(cfs_list_empty(&exp->exp_hp_rpcs)) failed

ptlrpc_server_handle_req_in() call ptlrpc_server_request_add()
and hp request can be added to export->exp_hp_rpcs, but reference
increase at ptlrpc_server_handle_request(). If class_disconnect() and
class_export_destroy() occures before ptlrpc_server_handle_request()
we got ASSERTION.

Signed-off-by: Alexander Boyko <alexander_boyko@xyratex.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Andriy Skulysh <andrew_skulysh@xyratex.com>
Xyratex-bug-id: MRP-649
Change-Id: I3d312c28481143b557d7987501c975c7e287885e
Reviewed-on: http://review.whamcloud.com/3855
Reviewed-by: Liang Zhen <liang@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>