net/smc: Add comment for smc_tx_pending

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>