net/smc: Fix error path in smc_init

If register_pernet_subsys success in smc_init,
we should cleanup it in case any other error.

Fixes: 64e28b52

 (net/smc: add pnet table namespace support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>