tcp: add ipv6_addr_v4mapped_loopback() helper

tcp_twsk_unique() has a hard coded assumption about ipv4 loopback
being 127/8

Lets instead use the standard ipv4_is_loopback() method,
in a new ipv6_addr_v4mapped_loopback() helper.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>