bpf: backtracking shouldn't clear outer frame R1-R5 for callbacks

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

bpf: backtracking shouldn't clear outer frame R1-R5 for callbacks

When processing calls to bpf_loop() verifier marks R1 (and R4) as
precise. R1 tracks loop iterations number and because of the
'callback_depth < R1' mechanics in check_helper_call() must be marked
precise. However, precision propagation for R1 was broken,
when bpf_loop() call was verified on a second iteration.

Consider the following verification trace:
- main: bpf_loop(nr_loops, callback ...)
- callback: BPF_EXIT
- main: bpf_loop(nr_loops, callback ...)
- ...

While the first visit of the call to bpf_loop() propagated R1
precision as expected, the second call to mark_chain_precision() in
the check_helper_call() set R1, but it was immediately reset when
backtrack_insn() processed preceding BPF_EXIT in the loop deleted in
this patch.

Because of that, the second visit of the call to bpf_loop() injected
checkpoint with R1 not marked as precise. Which could trick the
verifier into accepting unsafe programs. See the next patch for an
example of such program.

Commit is structured in a way to minimize conflicts when
'bpf' would be eventually merged with 'bpf-next'.