RDMA/rxe: Avoid reprocessing the current packet after the QP enters the error state

In the Linux kernel, the following vulnerability has been resolved:

RDMA/rxe: Avoid reprocessing the current packet after the QP enters the error state

When do_complete() finds the QP in the error state it returns
RESPST_CHK_RESOURCE. Before commit 49dc9c1f0c7e ("RDMA/rxe: Cleanup
reset state handling in rxe_resp.c") this was the flush loop:
check_resource() had an error-state branch that fetched each remaining
recv WQE and completed it with IB_WC_WR_FLUSH_ERR, without touching
the current packet. That commit removed the error-state branch from
check_resource() (draining is now done at rxe_receiver() entry) but
kept the do_complete() error-state return.

As a result, when a QP moves to the error state while a packet is
being completed - e.g. an rdma_cm disconnect racing with receive
processing - the responder state machine loops back into the request
processing chain with the already-completed packet still in hand:
check_resource() fetches a fresh recv WQE, execute()/send_data_in()
copies the same packet payload again, do_complete() posts another
IB_WC_SUCCESS CQE (qp->resp.status is still 0), and control returns
to the error-state check. The loop re-executes the same packet once
per posted recv WQE (observed: ~1000 duplicate IB_WC_SUCCESS
completions of one SEND, one per ~8us, matching the RQ occupancy)
until the RQ is exhausted, after which qp->resp.wqe is NULL and
send_data_in() dereferences it:

BUG: kernel NULL pointer dereference, address: 0000000000000014
Workqueue: rxe_wq do_work
RIP: copy_data+0x29/0x1f0
Call Trace:
send_data_in+0x25/0x50
rxe_receiver+0xf36/0x1dd0

The duplicate completions are indistinguishable from real receives to
the ULP. During an rds stress test, the message was accepted as new and
delivered the same datagram to user space hundreds of times, corrupting
the stream; any ULP that relies on RC exactly-once delivery is affected.

A live packet reaching the error-state check in do_complete() has
been executed and completed exactly once and must be consumed, not
re-processed. Return RESPST_CLEANUP for it (dequeue and free); keep
returning RESPST_CHK_RESOURCE for the pkt == NULL case.