net/smc: check variable before dereferencing in smc_close.c

smc->clcsock and smc->clcsock->sk are used before the check if they can
be dereferenced. Fix this by checking the variables first.

Fixes: a60a2b1e

 ("net/smc: reduce active tcp_listen workers")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>