net/smc: fix double kfree in smc_listen_work()

If smc_listen_rmda_finish() returns with an error, the storage
addressed by 'buf' is freed a second time.
Consolidate freeing under a common label and jump to that label.

Fixes: 6bb14e48 ("net/smc: dynamic allocation of CLC proposal buffer")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>