libbpf: Assume unsigned values for BTF_KIND_ENUM

libbpf: Assume unsigned values for BTF_KIND_ENUM

Currently, BTF_KIND_ENUM type doesn't record whether enum values should be
interpreted as signed or unsigned. In Linux, most enums are unsigned, though,
so interpreting them as unsigned matches real world better.

Change btf_dump test case to test maximum 32-bit value, instead of negative
value.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200303003233.3496043-3-andriin@fb.com