RDMA/rxe: Fix race condition in QP timer handlers

RDMA/rxe: Fix race condition in QP timer handlers

I encontered the following warning:
 WARNING: drivers/infiniband/sw/rxe/rxe_task.c:249 at rxe_sched_task+0x1c8/0x238 [rdma_rxe], CPU#0: swapper/0/0
...
  libsha1 [last unloaded: ip6_udp_tunnel]
 CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Tainted: G         C          6.19.0-rc5-64k-v8+ #37 PREEMPT
 Tainted: [C]=CRAP
 Hardware name: Raspberry Pi 4 Model B Rev 1.2
 Call trace:
  rxe_sched_task+0x1c8/0x238 [rdma_rxe] (P)
  retransmit_timer+0x130/0x188 [rdma_rxe]
  call_timer_fn+0x68/0x4d0
  __run_timers+0x630/0x888
...
 WARNING: drivers/infiniband/sw/rxe/rxe_task.c:38 at rxe_sched_task+0x1c0/0x238 [rdma_rxe], CPU#0: swapper/0/0
...
 WARNING: drivers/infiniband/sw/rxe/rxe_task.c:111 at do_work+0x488/0x5c8 [rdma_rxe], CPU#3: kworker/u17:4/93400
...
 refcount_t: underflow; use-after-free.
 WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x138/0x1a0, CPU#3: kworker/u17:4/93400

The issue is caused by a race condition between retransmit_timer() and
rxe_destroy_qp, leading to the Queue Pair's (QP) reference count dropping
to zero during timer handler execution.

It seems this warning is harmless because rxe_qp_do_cleanup() will flush
all pending timers and requests.

Example of flow causing the issue:

CPU0                                   CPU1
retransmit_timer() {
    spin_lock_irqsave
                           rxe_destroy_qp()
                            __rxe_cleanup()
                              __rxe_put() // qp->ref_count decrease to 0
                            rxe_qp_do_cleanup() {
    if (qp->valid) {
        rxe_sched_task() {
            WARN_ON(rxe_read(task->qp) <= 0);
        }
    }
    spin_unlock_irqrestore
}
                              spin_lock_irqsave
                              qp->valid = 0
                              spin_unlock_irqrestore
                            }

Ensure the QP's reference count is maintained and its validity is checked
within the timer callbacks by adding calls to rxe_get(qp) and corresponding
rxe_put(qp) after use.

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Fixes: d94671632572 ("RDMA/rxe: Rewrite rxe_task.c")
Link: https://patch.msgid.link/20260120074437.623018-1-lizhijian@fujitsu.com
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Leon Romanovsky <leon@kernel.org>