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

When BPF program drops pkts the sockmap logic 'eats' the packet and
updates copied_seq. In the PASS case where the sk_buff is accepted
we update copied_seq from recvmsg path so we need a new test to
handle the drop case.

Original patch series broke this resulting in

test_sockmap_skb_verdict_fionread:PASS:ioctl(FIONREAD) error 0 nsec
test_sockmap_skb_verdict_fionread:FAIL:ioctl(FIONREAD) unexpected ioctl(FIONREAD): actual 1503041772 != expected 256

After updated patch with fix.

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/20230523025618.113937-14-john.fastabend@gmail.com