ipv6: Use ipv6_only_sock() helper in condition.

This patch replaces some sk_ipv6only tests with ipv6_only_sock().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>