selftests/bpf: Use -1 as token_fd in map create failure test

Because 0xFF can be an open BPF token fd in the test runner that will fail
test_invalid_token_fd(), change token_fd from 0xFF to -1 to avoid such
test failure.

Fixes: f675483cac ("selftests/bpf: Add tests to verify map create failure log")
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20260518145446.6794-5-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -306,7 +306,7 @@ static void test_invalid_token_fd(void)
 	const char *msg = "Invalid map_token_fd.\n";
 	LIBBPF_OPTS(bpf_map_create_opts, opts,
 		    .map_flags = BPF_F_TOKEN_FD,
-		    .token_fd = 0xFF,
+		    .token_fd = -1,
 	);
 
 	test_map_create_array(&opts, msg);