net/sock: Introduce trace_sk_data_ready()

As suggested by Cong, introduce a tracepoint for all ->sk_data_ready()
callback implementations.  For example:

<...>
  iperf-609  [002] .....  70.660425: sk_data_ready: family=2 protocol=6 func=sock_def_readable
  iperf-609  [002] .....  70.660436: sk_data_ready: family=2 protocol=6 func=sock_def_readable
<...>

Suggested-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>