tcp: use EXPORT_IPV6_MOD[_GPL]()

tcp: use EXPORT_IPV6_MOD[_GPL]()

Use EXPORT_IPV6_MOD[_GPL]() for symbols that don't need
to be exported unless CONFIG_IPV6=m

tcp_hashinfo and tcp_openreq_init_rwin() are no longer
used from any module anyway.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Link: https://patch.msgid.link/20250212132418.1524422-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>