selftests/bpf: Fix UBSan complaint about signed __int128 overflow

Test is using __int128 variable as unsigned and highest order bit can be
set to 1 after bit shift. Use unsigned __int128 explicitly and prevent
UBSan from complaining.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-8-andrii@kernel.org

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

@@ -323,7 +323,7 @@ static void test_btf_dump_int_data(struct btf *btf, struct btf_dump *d,
 				   char *str)
 {
 #ifdef __SIZEOF_INT128__
-	__int128 i = 0xffffffffffffffff;
+	unsigned __int128 i = 0xffffffffffffffff;
 
 	/* this dance is required because we cannot directly initialize
 	 * a 128-bit value to anything larger than a 64-bit value.