Skip test setup if device does not support bpf

For older devices that doesn't have bpf support, skip the whole test so
we don't get false alarms. Fixed some compiler warning as well.

Test: ./libnetdbpf_test/libnetdbpf_test
Bug: 119526856
Change-Id: I5a520c3157510372c016c8dee65785eddd8ae431
1 file changed