bpf: Remove __btf_name_valid() and change to btf_name_valid_identifier()

__btf_name_valid() can be completely replaced with
btf_name_valid_identifier, and since most of the time you already call
btf_name_valid_identifier instead of __btf_name_valid , it would be
appropriate to rename the __btf_name_valid function to
btf_name_valid_identifier and remove __btf_name_valid.

Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20240807143110.181497-1-aha310510@gmail.com