bpf: verifier: Rename kernel_type_name helper to btf_type_name

bpf: verifier: Rename kernel_type_name helper to btf_type_name

kernel_type_name was introduced in commit 9e15db66136a ("bpf: Implement accurate raw_tp context access via BTF")
with type signature:

  const char *kernel_type_name(u32 id)

At that time the function used global btf_vmlinux BTF for all id lookups. Later,
in commit 22dc4a0f5ed1 ("bpf: Remove hard-coded btf_vmlinux assumption from BPF verifier"),
the type signature was changed to:

  static const char *kernel_type_name(const struct btf* btf, u32 id)

With the btf parameter used for lookups instead of global btf_vmlinux.

The helper will function as expected for type name lookup using non-kernel BTFs,
and will be used for such in further patches in the series. Let's rename it to
avoid incorrect assumptions that might arise when seeing the current name.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230309180111.1618459-2-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>