net/smc: fix a memory leak in smc_sysctl_net_exit()

Recently added smc_sysctl_net_exit() forgot to free
the memory allocated from smc_sysctl_net_init()
for non initial network namespace.

Fixes: 462791bb

 ("net/smc: add sysctl interface for SMC")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Tony Lu <tonylu@linux.alibaba.com>
Cc: Dust Li <dust.li@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>