net/smc: cleanup buffer usage in smc_listen_work()

coccinelle informs about
net/smc/af_smc.c:1770:10-11: WARNING: opportunity for kzfree/kvfree_sensitive

Its not that kzfree() would help here, the memset() is done to prepare
the buffer for another socket receive.
Fix that warning message by reordering the calls, while at it eliminate
the unneeded variable cclc2 and use sizeof(*buf) as above in the same
function. No functional changes.

Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>