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

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: f675483cac1d ("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>