selftests/bpf: Convert test_bpffs to ASSERT macros

Remove usage of deprecated CHECK macros.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211028094724.59043-4-lmb@cloudflare.com