tcp: refactor tcp_retransmit_timer()

It appears linux-4.14 stable needs a backport of commit
88f8598d ("tcp: exit if nothing to retransmit on RTO timeout")

Since tcp_rtx_queue_empty() is not in pre 4.15 kernels,
let's refactor tcp_retransmit_timer() to only use tcp_rtx_queue_head()

I will provide to stable teams the squashed patches.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>