CVE-2026-23415 futex: Fix UaF between futex_key_to_node_opt() and vma_replace_policy()

In the Linux kernel, the following vulnerability has been resolved: futex: Fix UaF between futexkeytonodeopt and vmareplacepolicy During futexkeytonodeopt execution, vma-vmpolicy is read under speculative mmap lock and RCU. Concurrently, mbind may call vmareplacepolicy which frees the old mempoli...