bpf: Tag nullable kfunc pointer args with PTR_MAYBE_NULL

Now that get_kfunc_ptr_arg_type() classifies a kfunc pointer argument
from its BTF alone, express a nullable argument by OR-ing PTR_MAYBE_NULL
into the classified type, and resolve a NULL register after
classification instead of before it.

Previously check_kfunc_args() short-circuited a nullable argument passed
a NULL register with a continue placed before get_kfunc_ptr_arg_type(),
so the NULL never reached classification. That kept a register-state
decision (bpf_register_is_null()) ahead of the BTF-based classification.

This mirrors how helper arguments carry PTR_MAYBE_NULL in their
bpf_arg_type and is a step toward describing kfuncs with a bpf_func_proto:
the nullability now travels with the per-argument classification, so it is
captured when the prototype is generated at add-call time.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260801074633.1595644-17-ameryhung@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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