ipv4: tcp: clean up tcp_v4_early_demux()

Use same header helpers than tcp_v6_early_demux() because they
are a bit faster, and as they make IPv4/IPv6 versions look
the same.

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