bpf: btf: implement btf_name_valid_identifier()

Function btf_name_valid_identifier() have been implemented in
bpf-next commit 2667a262 ("bpf: btf: Add BTF_KIND_FUNC and
BTF_KIND_FUNC_PROTO"). Backport this function so later patch
can use it.

Fixes: 69b693f0

 ("bpf: btf: Introduce BPF Type Format (BTF)")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>