selftests/bpf: Change NAME_NTH/IS_NAME_NTH for BTF_KIND_TAG format

BTF_KIND_TAG ELF format has a component_idx which might have value -1.
test_btf may confuse it with common_type.name as NAME_NTH checkes
high 16bit to be 0xffff. Change NAME_NTH high 16bit check to be
0xfffe so it won't confuse with component_idx.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210914223041.248009-1-yhs@fb.com

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

@@ -39,8 +39,8 @@ static bool always_log;
 #define BTF_END_RAW 0xdeadbeef
 #define NAME_TBD 0xdeadb33f
 
-#define NAME_NTH(N) (0xffff0000 | N)
-#define IS_NAME_NTH(X) ((X & 0xffff0000) == 0xffff0000)
+#define NAME_NTH(N) (0xfffe0000 | N)
+#define IS_NAME_NTH(X) ((X & 0xffff0000) == 0xfffe0000)
 #define GET_NAME_NTH_IDX(X) (X & 0x0000ffff)
 
 #define MAX_NR_RAW_U32 1024