selftests/bpf: Add selftests for meta

Add a bigger batch of test coverage to assert correct operation of
meta device and its BPF program management:

  # ./vmtest.sh -- ./test_progs -t tc_meta
  [...]
  ./test_progs -t tc_meta
  [    1.211407] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.211805] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  [    1.271692] tsc: Refined TSC clocksource calibration: 3407.989 MHz
  [    1.274015] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x311fc9c9451, max_idle_ns: 440795361646 ns
  [    1.275241] clocksource: Switched to clocksource tsc
  #255     tc_meta_basic:OK
  #256     tc_meta_device:OK
  #257     tc_meta_multi_links:OK
  #258     tc_meta_multi_opts:OK
  #259     tc_meta_neigh_links:OK
  Summary: 5/0 PASSED, 0 SKIPPED, 0 FAILED
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/config

@@ -43,6 +43,7 @@ CONFIG_IPV6_TUNNEL=y
 CONFIG_KEYS=y
 CONFIG_LIRC=y
 CONFIG_LWTUNNEL=y
+CONFIG_META=y
 CONFIG_MODULE_SIG=y
 CONFIG_MODULE_SRCVERSION_ALL=y
 CONFIG_MODULE_UNLOAD=y

tools/testing/selftests/bpf/prog_tests/tc_helpers.h

@@ -4,6 +4,10 @@
 #define TC_HELPERS
 #include <test_progs.h>
 
+#ifndef loopback
+# define loopback 1
+#endif
+
 static inline __u32 id_from_prog_fd(int fd)
 {
 	struct bpf_prog_info prog_info = {};