bpf, selftests: Add various BPF tc link selftests

    [ Commit msg tbd ]

    todo:
    - selftest with old/legacy tc api
    - selftest with chaining & termination

      # ./vmtest.sh -- ./test_progs -t tc_link
      [...]
      [    1.486605] clocksource: Switched to clocksource tsc
      ./test_progs -t tc_link
      [    1.542394] bpf_testmod: loading out-of-tree module taints kernel.
      [    1.543803] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
      torvalds#179     tc_link_base:OK
      torvalds#180     tc_link_mix:OK
      torvalds#181     tc_link_opts:OK
      torvalds#182     tc_link_run:OK
      Summary: 4/0 PASSED, 0 SKIPPED, 0 FAILED
      [...]

Signed-off-by: Daniel Borkmann <[email protected]>