bpf: Fix bpf_check_mod_kfunc_call for built-in modules

When module registering its set is built-in, THIS_MODULE will be NULL,
hence we cannot return early in case owner is NULL.

Fixes: 14f267d9 ("bpf: btf: Introduce helpers for dynamic BTF set registration")
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/20211122144742.477787-3-memxor@gmail.com