bpf/btf: Use btf_type_str() whenever possible

We have btf_type_str().  Use it whenever possible in btf.c, instead of
"btf_kind_str[BTF_INFO_KIND(t->info)]".

Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Link: https://lore.kernel.org/r/20220916202800.31421-1-yepeilin.cs@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>