selftests/bpf: Add a selftest for invalid func btf with btf decl_tag

Added a selftest similar to [1] which exposed a kernel bug.
Without the fix in the previous patch, the similar kasan error will appear.

  [1] https://lore.kernel.org/bpf/0000000000009b6eaa05d71a8c06@google.com/Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220203191732.742285-1-yhs@fb.com

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

@@ -3938,6 +3938,25 @@ static struct btf_raw_test raw_tests[] = {
 	.btf_load_err = true,
 	.err_str = "Invalid component_idx",
 },
+{
+	.descr = "decl_tag test #15, func, invalid func proto",
+	.raw_types = {
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),	/* [1] */
+		BTF_DECL_TAG_ENC(NAME_TBD, 3, 0),		/* [2] */
+		BTF_FUNC_ENC(NAME_TBD, 8),			/* [3] */
+		BTF_END_RAW,
+	},
+	BTF_STR_SEC("\0tag\0func"),
+	.map_type = BPF_MAP_TYPE_ARRAY,
+	.map_name = "tag_type_check_btf",
+	.key_size = sizeof(int),
+	.value_size = 4,
+	.key_type_id = 1,
+	.value_type_id = 1,
+	.max_entries = 1,
+	.btf_load_err = true,
+	.err_str = "Invalid type_id",
+},
 {
 	.descr = "type_tag test #1",
 	.raw_types = {