RDMA/hns: Support drain SQ and RQ

RDMA/hns: Support drain SQ and RQ

Some ULPs, e.g. rpcrdma, rely on drain_qp() to ensure all outstanding
requests are completed before releasing related memory. If drain_qp()
fails, ULPs may release memory directly, and in-flight WRs may later be
flushed after the memory is freed, potentially leading to UAF.

drain_qp() failures can happen when HW enters an error state or is
reset. Add support to drain SQ and RQ in such cases by posting a
fake WR during reset, so the driver can process all remaining WRs in
sequence and generate corresponding completions.

Always invoke comp_handler() in drain process to ensure completions
are not lost under concurrency (e.g. concurrent post_send() and
reset, or QPs created during reset). If the CQ is already processed,
cancel any already scheduled comp_handler() to avoid concurrency
issues.

Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Link: https://patch.msgid.link/20260108113032.856306-1-huangjunxian6@hisilicon.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>