selftests/bpf: Test attaching a bpf qdisc with incomplete operators

Implement .destroy in bpf_fq and bpf_fifo as it is now mandatory.

Test attaching a bpf qdisc with a missing operator .init. This is not
allowed as bpf qdisc qdisc_watchdog_cancel() could have been called with
an uninitialized timer.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>