Merge branch 'bpf-arg-tracking-for-imprecise-multi-offset-bpf_st-stx'

Eduard Zingerman says:

====================
bpf: arg tracking for imprecise/multi-offset BPF_ST/STX

When the static arg tracking analysis encounters a store through a
pointer with imprecise or multi-offset destination, it must use weak
updates (join) instead of strong updates (overwrite) for the affected
at_stack slots. At runtime only one slot is actually written; the
others retain their old values.

Two cases are addressed:
- BPF_STX, handled by spill_to_stack(). It was gated on
  `dst_is_local_fp = (frame == depth)`, which missed ARG_IMPRECISE
  pointers entirely.
- BPF_ST, handled by clear_stack_for_all_offs(). It delegates to
  clear_overlapping_stack_slots() which unconditionally set
  `at_stack[i] = none`. Change to `at_stack[i] = join(old, none)`
  when multiple candidate slots exist (cnt != 1), so that untouched
  slots preserve their tracked values.

No veristat diff compared to current master when tested on selftests,
sched_ext, cilium and a set of Meta internal programs.

This addresses issues reported by sashiko for patch #7 in [1].

[1] https://sashiko.dev/#/patchset/20260410-patch-set-v4-0-5d4eecb343db%40gmail.com

Changelog:
v2 -> v3:
- Use check_add_overflow() in arg_add() (Alexei).
- Add missing fixes tag (CI bot).
- Remove unused __imm in the selftest (sashiko).
v1 -> v2:
- Delete the OFF_IMPRECISE constant, always rely on
  arg_track->cnt == 0 as a marker the offset is imprecise.
  (Alexei).
- Squash all patches together to simplify backporting to
  'bpf' branch (Alexei).

v1: https://lore.kernel.org/bpf/20260413-stacklive-fixes-v1-0-9f48a9999d6e@gmail.com/T/
v2: https://lore.kernel.org/bpf/20260413-stacklive-fixes-v2-0-ff91c4f8d273@gmail.com/T/
---
====================

Link: https://patch.msgid.link/20260413-stacklive-fixes-v2-0-398e126e5cf3@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>