LU-898 ptlrpc: fix ptlrpc request race.

LU-898 ptlrpc: fix ptlrpc request race.

Allow request reorder from export, only if request has been
added to the list.

Race condition with req error handle at ptlrpc_server_handle_req_in().
1. req is added to rq_export->exp_queued_rpc by ptlrpc_hpreq_init()
2. ptlrpc_server_request_add() returns error ( ost_validate_obdo() whith
one of this condition !(fid_seq_is_rsvd(oa->o_seq) ||
fid_seq_is_idif(oa->o_seq))
3. ptlrpc_server_drop_request(req) drops request (disconnect export),
but req is in rq_export->exp_queued_rpc
4. ldlm_server_blocking_ast handle rq_export->exp_queued_rpc and fail
because req->rq_export is NULL.
Fix allows request reorder from export only if request has been added to
the list. So workaround is not need for situation when request from export
was processed before ptlrpc_server_request_add.

Signed-off-by: Alexander Boyko <alexander_boyko@xyratex.com>
Reviewed-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Reviewed-by: Denis Kondratenko <denis_kondratenko@xyratex.com>
Reviewed-by: Andrew Perepechko <Andrew_Perepechko@xyratex.com>
Xyratex-bug-id: MRP-284
Change-Id: I5f763b3c4f19b6af5f803b50b43a5570dab3dc76
Reviewed-on: http://review.whamcloud.com/1799
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Reviewed-by: Denis Kondratenko <Denis_Kondratenko@xyratex.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>