bpf: add perf event notificaton support for sock_ops

This patch allows eBPF programs that use sock_ops to send perf
based event notifications using bpf_perf_event_output(). Our main
use case for this is the following:

  We would like to monitor some subset of TCP sockets in user-space,
  (the monitoring application would define 4-tuples it wants to monitor)
  using TCP_INFO stats to analyze reported problems. The idea is to
  use those stats to see where the bottlenecks are likely to be ("is
  it application-limited?" or "is there evidence of BufferBloat in
  the path?" etc).

  Today we can do this by periodically polling for tcp_info, but this
  could be made more efficient if the kernel would asynchronously
  notify the application via tcp_info when some "interesting"
  thresholds (e.g., "RTT variance > X", or "total_retrans > Y" etc)
  are reached. And to make this effective, it is better if
  we could apply the threshold check *before* constructing the
  tcp_info netlink notification, so that we don't waste resources
  constructing notifications that will be discarded by the filter.

This work solves the problem by adding perf event based notification
support for sock_ops. The eBPF program can thus be designed to apply
any desired filters to the bpf_sock_ops and trigger a perf event
notification based on the evaluation from the filter. The user space
component can use these perf event notifications to either read any
state managed by the eBPF program, or issue a TCP_INFO netlink call
if desired.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>