bpf: Change bpf_kallsyms_lookup_name size type to ARG_CONST_SIZE_OR_ZERO

Andrii mentioned in [0] that switching to ARG_CONST_SIZE_OR_ZERO lets
user avoid having to prove that string size at runtime is not zero and
helps with not having to supress clang optimizations.

  [0]: https://lore.kernel.org/bpf/CAEf4BzZa_vhXB3c8atNcTS6=krQvC25H7K7c3WWZhM=27ro=Wg@mail.gmail.com



Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211122235733.634914-2-memxor@gmail.com