selftest/bpf: Add test for af_vsock poll()

Verify that vsock's poll() notices when sk_psock::ingress_msg isn't empty.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20241118-vsock-bpf-poll-close-v1-2-f1b9669cacdc@rbox.co


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>