bpf, sockmap: Test shutdown() correctly exits epoll and recv()=0

When session gracefully shutdowns epoll needs to wake up and any recv()
readers should return 0 not the -EAGAIN they previously returned.

Note we use epoll instead of select to test the epoll wake on shutdown
event as well.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20230523025618.113937-12-john.fastabend@gmail.com