Merge branch 'fix-missing-process_iter_arg-type-check'

Kumar Kartikeya Dwivedi says:

====================
Fix missing process_iter_arg type check

I am taking over Tao's earlier patch set that can be found at [0], after
an offline discussion. The bug reported in that thread is that
process_iter_arg missed a reg->type == PTR_TO_STACK check. Fix this by
adding it in, and also address comments from Andrii on the earlier
attempt. Include more selftests to ensure the error is caught.

  [0]: https://lore.kernel.org/bpf/20241107214736.347630-1-tao.lyu@epfl.ch

Changelog:
----------
v1 -> v2:
v1: https://lore.kernel.org/bpf/20241127230147.4158201-1-memxor@gmail.com
====================

Link: https://patch.msgid.link/20241203000238.3602922-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>