bpf: Reject resilient lock operations in rbtree callbacks

__bpf_rbtree_add() keeps parent and link pointers live across calls to the
program-supplied comparison callback. The verifier therefore requires the
root's lock to remain held throughout the callback.

The helper path enforces this rule for bpf_spin_lock() and
bpf_spin_unlock(), but the resilient lock kfunc argument path does not.
Since resilient locks may protect BPF rbtree roots, a callback can release
the root lock and let another CPU remove and free the node referenced by
the in-progress tree walk. The walk then resumes using freed pointers.

Reject resilient lock kfuncs in an rbtree comparison callback, matching
the existing policy for the spin lock helpers. Resilient-lock-protected
trees remain valid when their comparison callbacks leave lock state alone.

Fixes: 0de2046137f9 ("bpf: Implement verifier support for rqspinlock")
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/20260903144433.1716731-6-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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