libbpf: Fix an unsigned < 0 bug

Andrii reported a bug with the following information:

  2859 	if (enum64_placeholder_id == 0) {
  2860 		enum64_placeholder_id = btf__add_int(btf, "enum64_placeholder", 1, 0);
  >>>     CID 394804:  Control flow issues  (NO_EFFECT)
  >>>     This less-than-zero comparison of an unsigned value is never true. "enum64_placeholder_id < 0U".
  2861 		if (enum64_placeholder_id < 0)
  2862 			return enum64_placeholder_id;
  2863    	...

Here enum64_placeholder_id declared as '__u32' so enum64_placeholder_id < 0
is always false. Declare enum64_placeholder_id as 'int' in order to capture
the potential error properly.

Fixes: f2a62588 ("libbpf: Add enum64 sanitization")
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220613054314.1251905-1-yhs@fb.com