bpf: Fix wrong reg type conversion in release_reference()

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

bpf: Fix wrong reg type conversion in release_reference()

Some helper functions will allocate memory. To avoid memory leaks, the

verifier requires the eBPF program to release these memories by calling

the corresponding helper functions.

When a resource is released, all pointer registers corresponding to the

resource should be invalidated. The verifier use release_references() to

do this job, by apply __mark_reg_unknown() to each relevant register.

It will give these registers the type of SCALAR_VALUE. A register that

will contain a pointer value at runtime, but of type SCALAR_VALUE, which

may allow the unprivileged user to get a kernel pointer by storing this

register into a map.

Using __mark_reg_not_init() while NOT allow_ptr_leaks can mitigate this

problem.