net/smc: make smc_hash_sk/smc_unhash_sk static

smc_hash_sk and smc_unhash_sk are only used in af_smc.c, so make them
static and remove the output symbol. They can be called under the path
.prot->hash()/unhash().

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Wen Gu <guwen@linux.alibaba.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>