selftests/bpf: check states pruning for deeply nested iterator

selftests/bpf: check states pruning for deeply nested iterator

A test case with ridiculously deep bpf_for() nesting and
a conditional update of a stack location.

Consider the innermost loop structure:

	1: bpf_for(o, 0, 10)
	2:	if (unlikely(bpf_get_prandom_u32()))
	3:		buf[0] = 42;
	4: <exit>

Assuming that verifier.c:clean_live_states() operates w/o change from
the previous patch (e.g. as on current master) verification would
proceed as follows:
- at (1) state {buf[0]=?,o=drained}:
  - checkpoint
  - push visit to (2) for later
- at (4) {buf[0]=?,o=drained}
- pop (2) {buf[0]=?,o=active}, push visit to (3) for later
- at (1) {buf[0]=?,o=active}
  - checkpoint
  - push visit to (2) for later
- at (4) {buf[0]=?,o=drained}
- pop (2) {buf[0]=?,o=active}, push visit to (3) for later
- at (1) {buf[0]=?,o=active}:
  - checkpoint reached, checkpoint's branch count becomes 0
  - checkpoint is processed by clean_live_states() and
    becomes {o=active}
- pop (3) {buf[0]=42,o=active}
- at (1), {buf[0]=42,o=active}
  - checkpoint
  - push visit to (2) for later
- at (4) {buf[0]=42,o=drained}
- pop (2) {buf[0]=42,o=active}, push visit to (3) for later
- at (1) {buf[0]=42,o=active}, checkpoint reached
- pop (3) {buf[0]=42,o=active}
- at (1) {buf[0]=42,o=active}:
  - checkpoint reached, checkpoint's branch count becomes 0
  - checkpoint is processed by clean_live_states() and
    becomes {o=active}
- ...

Note how clean_live_states() converted the checkpoint
{buf[0]=42,o=active} to {o=active} and it can no longer be matched
against {buf[0]=<any>,o=active}, because iterator based states
are compared using stacksafe(... RANGE_WITHIN), that requires
stack slots to have same types. At the same time there are
still states {buf[0]=42,o=active} pushed to DFS stack.

This behaviour becomes exacerbated with multiple nesting levels,
here are veristat results:
- nesting level 1: 69 insns
- nesting level 2: 258 insns
- nesting level 3: 900 insns
- nesting level 4: 4754 insns
- nesting level 5: 35944 insns
- nesting level 6: 312558 insns
- nesting level 7: 1M limit

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250215110411.3236773-5-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>