bpf: Clear NON_OWN_REF after RCU protection ends

A local kptr load of an object containing a graph node is marked MEM_RCU
and NON_OWN_REF while protected by RCU. When the last RCU read-side critical
section ends, invalidate_rcu_protected_refs() removes MEM_RCU and marks the
pointer PTR_UNTRUSTED, but leaves NON_OWN_REF set.

The stale flag lets graph kfunc argument checks continue treating the
pointer as a live borrowed reference. In particular, bpf_rbtree_remove()
can accept a pointer after its protection ended and return it as a new
owning reference, even though the object may already have been freed.

Clear NON_OWN_REF when an RCU-protected pointer is demoted. A spin lock also
provides implicit RCU protection, so invalidate non-owning references before
demoting RCU-protected pointers when releasing the lock. Otherwise the
demotion would clear the flag before invalidate_non_owning_refs() can find
and invalidate those aliases.

The demoted pointer remains available for fault-protected reads. Exempt such
reads from the allocated-object reference-state assertion; writes through a
fault-prone pointer are already rejected, and bpf_may_fault_on_deref() makes
the surviving loads use BPF_PROBE_MEM.

Fixes: 1b12171533a9 ("bpf: Mark direct ld of stashed bpf_{rb,list}_node as non-owning ref")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-6-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>