selftests/bpf: Enable tc verbose mode for test_sk_assign

Currently test_sk_assign failed verifier with llvm11/llvm12.
During debugging, I found the default verifier output is
truncated like below
  Verifier analysis:

  Skipped 2200 bytes, use 'verb' option for the full verbose log.
  [...]
  off=23,r=34,imm=0) R5=inv0 R6=ctx(id=0,off=0,imm=0) R7=pkt(id=0,off=0,r=34,imm=0) R10=fp0
  80: (0f) r7 += r2
  last_idx 80 first_idx 21
  regs=4 stack=0 before 78: (16) if w3 == 0x11 goto pc+1
when I am using "./test_progs -vv -t assign".

The reason is tc verbose mode is not enabled.

This patched enabled tc verbose mode and the output looks like below
  Verifier analysis:

  0: (bf) r6 = r1
  1: (b4) w0 = 2
  2: (61) r1 = *(u32 *)(r6 +80)
  3: (61) r7 = *(u32 *)(r6 +76)
  4: (bf) r2 = r7
  5: (07) r2 += 14
  6: (2d) if r2 > r1 goto pc+61
   R0_w=inv2 R1_w=pkt_end(id=0,off=0,imm=0) R2_w=pkt(id=0,off=14,r=14,imm=0)
  ...
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200824222807.100200-1-yhs@fb.com

tools/testing/selftests/bpf/prog_tests/sk_assign.c

@@ -49,7 +49,7 @@ configure_stack(void)
 	sprintf(tc_cmd, "%s %s %s %s", "tc filter add dev lo ingress bpf",
 		       "direct-action object-file ./test_sk_assign.o",
 		       "section classifier/sk_assign_test",
-		       (env.verbosity < VERBOSE_VERY) ? " 2>/dev/null" : "");
+		       (env.verbosity < VERBOSE_VERY) ? " 2>/dev/null" : "verbose");
 	if (CHECK(system(tc_cmd), "BPF load failed;",
 		  "run with -vv for more info\n"))
 		return false;