netfilter: xt_socket: prepare for TCP_NEW_SYN_RECV support

TCP request socks soon will be visible in ehash table.

xt_socket will be able to match them, but first we need
to make sure to not consider them as full sockets.

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