netfilter: Decrease code duplication regarding transparent socket option

There is a function in include/net/netfilter/nf_socket.h to decide if a
socket has IP(V6)_TRANSPARENT socket option set or not. However this
does the same as inet_sk_transparent() in include/net/tcp.h

include/net/tcp.h:1733
/* This helper checks if socket has IP_TRANSPARENT set */
static inline bool inet_sk_transparent(const struct sock *sk)
{
	switch (sk->sk_state) {
	case TCP_TIME_WAIT:
		return inet_twsk(sk)->tw_transparent;
	case TCP_NEW_SYN_RECV:
		return inet_rsk(inet_reqsk(sk))->no_srccheck;
	}
	return inet_sk(sk)->transparent;
}

tproxy_sk_is_transparent has also been refactored to use this function
instead of reimplementing it.

Signed-off-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>