net: create tcp_gro_lookup helper function

This pulls the flow port matching out of tcp_gro_receive, so that it can be
reused for the next change, which adds the TCP fraglist GRO heuristic.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>