net/smc: remove unreferenced header in smc_loopback.h file

Because linux/err.h is unreferenced in smc_loopback.h file, so
remove it.

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