Local service hang via shmem truncate retry loop

Infinite-loop denial of service in Linux Kernel 6.12 allows local users to hang a truncation operation involving a large swap entry. The shmem truncation path passes an index from the middle of a large swap entry to shmem_free_swap(), then retries that unchanged index after a base-index mismatch, so find_get_entries() returns the same entry forever. Reaching the loop requires local low-privilege access, a large swap entry, and the truncation race; the resulting impact is an availability hang.