bpf, sockmap: Test FIONREAD returns correct bytes in rx buffer

A bug was reported where ioctl(FIONREAD) returned zero even though the
socket with a SK_SKB verdict program attached had bytes in the msg
queue. The result is programs may hang or more likely try to recover,
but use suboptimal buffer sizes.

Add a test to check that ioctl(FIONREAD) returns the correct number of
bytes.

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/20230523025618.113937-13-john.fastabend@gmail.com