ipv6: Remove redundant dst NULL check in ip6_dst_check

As the only path leading to ip6_dst_check makes an indirect call
through dst->ops, dst cannot be NULL in ip6_dst_check.

This patch removes this check in case it misleads people who
come across this code.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>