bpf: Reject MEM_ALLOC BTF accesses past object bounds

In the Linux kernel, the following vulnerability has been resolved: bpf: Reject MEM_ALLOC BTF accesses past object bounds BTF struct walks relax the struct-size check for accesses through a trailing flexible array. That is valid for ordinary BTF type walking, but PTR_TO_BTF_ID | MEM_ALLOC values point to objects allocated with the static BTF type size. When walking a MEM_ALLOC object, reject the access before applying the flexible-array relaxation if the access range extends past the struct size. Apply the same policy to struct ID matching so kfunc and kptr type checks do not walk past the allocated object bounds either.