tcp: add trace event class tcp_event_sk_skb

Introduce event class tcp_event_sk_skb for tcp tracepoints that
have arguments sk and skb.

Existing tracepoint trace_tcp_retransmit_skb() falls into this class.
This patch rewrites the definition of trace_tcp_retransmit_skb() with
tcp_event_sk_skb.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>