net/smc: Add comment for smc_tx_pending

mainline inclusion
from mainline-v5.18-rc1
commit 2e13bde1
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I78OQ2
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/net/smc?id=2e13bde1315373cf7e9fb7dc4330d0d2a6bd5417



--------------------------------

The previous patch introduces a lock-free version of smc_tx_work() to
solve unnecessary lock contention, which is expected to be held lock.
So this adds comment to remind people to keep an eye out for locks.

Suggested-by: Stefan Raspl <raspl@linux.ibm.com>
Signed-off-by: Tony Lu <tonylu@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Litao Jiao <jiaolitao@sangfor.com.cn>