Indefinite task hang via unsynchronized futex state reads

Race condition in the Linux kernel allows local users to block a futex-resizer task indefinitely during private hash resizing. futex_pivot_pending() reads hash_new and the current hash without serialization, so a concurrent pivot can combine pre-pivot and post-pivot state and miss the final-reference wake. Reachability requires a custom private hash resize racing another futex task; the blocked task can hang indefinitely, and a configured hung-task detector can panic the kernel.