tcp: Save v4 address as v4-mapped-v6 in inet_bind2_bucket.v6_rcv_saddr.

In bhash2, IPv4/IPv6 addresses are saved in two union members,
which complicate address checks in inet_bind2_bucket_addr_match()
and inet_bind2_bucket_match_addr_any() considering uninitialised
memory and v4-mapped-v6 conflicts.

Let's simplify that by saving IPv4 address as v4-mapped-v6 address
and defining tb2.rcv_saddr as tb2.v6_rcv_saddr.s6_addr32[3].

Then, we can compare v6 address as is, and after checking v4-mapped-v6,
we can compare v4 address easily.  Also, we can remove tb2->family.

Note these functions will be further refactored in the next patch.

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