selftests/bpf: Adapt assert_mprog_count to always expect 0 count

Simplify __assert_mprog_count() to remove the -ENOENT corner case as the
bpf_prog_query() now returns 0 when no bpf_mprog is attached. This also
allows to convert a few test cases from using raw __assert_mprog_count()
over to plain assert_mprog_count() helper.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20231006220655.1653-5-daniel@iogearbox.net


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>