Both tcp_data_ready() and tcp_stream_is_readable() share the same logic.

Add tcp_epollin_ready() helper to avoid duplication.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Wei Wang <weiwan@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>