bpf: sockmap indicate sock events to listeners

After userspace pushes sockets into a sockmap it may not be receiving
data (assuming stream_{parser|verdict} programs are attached). But, it
may still want to manage the socks. A common pattern is to poll/select
for a POLLRDHUP event so we can close the sock.

This patch adds the logic to wake up these listeners.

Also add TCP_SYN_SENT to the list of events to handle. We don't want
to break the connection just because we happen to be in this state.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>