selftests/bpf: Allow for iteration over multiple states

Add parentheses around loopback address check to fix up logic and make
the socket state filter configurable for the TCP socket iterators.
Iterators can skip the socket state check by setting ss to 0.

Signed-off-by: Jordan Rife <jordan@jrife.io>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>