bpf: reject direct access to nullable PTR_TO_BUF pointers

In the Linux kernel, the following vulnerability has been resolved:

bpf: reject direct access to nullable PTR_TO_BUF pointers

check_mem_access() matches PTR_TO_BUF via base_type() which strips

PTR_MAYBE_NULL, allowing direct dereference without a null check.

Map iterator ctx->key and ctx->value are PTR_TO_BUF | PTR_MAYBE_NULL.

On stop callbacks these are NULL, causing a kernel NULL dereference.

Add a type_may_be_null() guard to the PTR_TO_BUF branch, matching the

existing PTR_TO_BTF_ID pattern.