net/smc: fix NULL sndbuf_desc in smc_cdc_tx_handler()

net/smc: fix NULL sndbuf_desc in smc_cdc_tx_handler()

When performing a stress test on SMC-R by rmmod mlx5_ib driver
during the wrk/nginx test, we found that there is a probability
of triggering a panic while terminating all link groups.

This issue dues to the race between smc_smcr_terminate_all()
and smc_buf_create().

			smc_smcr_terminate_all

smc_buf_create
/* init */
conn->sndbuf_desc = NULL;
...

			__smc_lgr_terminate
				smc_conn_kill
					smc_close_abort
						smc_cdc_get_slot_and_msg_send

			__softirqentry_text_start
				smc_wr_tx_process_cqe
					smc_cdc_tx_handler
						READ(conn->sndbuf_desc->len);
						/* panic dues to NULL sndbuf_desc */

conn->sndbuf_desc = xxx;

This patch tries to fix the issue by always to check the sndbuf_desc
before send any cdc msg, to make sure that no null pointer is
seen during cqe processing.

Fixes: 0b29ec643613 ("net/smc: immediate termination for SMCR link groups")
Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>
Reviewed-by: Tony Lu <tonylu@linux.alibaba.com>
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Link: https://lore.kernel.org/r/1678263432-17329-1-git-send-email-alibuda@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>