Remote service disruption via NFSv4 grace race

Race condition in Linux kernel NFSv4 server code allows remote attackers to corrupt memory and crash the host during grace-period recovery. The nfsd4_end_grace function reads and writes a plain grace_ended Boolean without serialization, so a laundromat worker and RECLAIM_COMPLETE thread can both drain reclaim_str_hashtbl and double-free its entries. Exploitation requires an active NFSv4 server with reclaim records and a race at grace completion; the bounded consequence is kernel-memory corruption, use-after-free, and service interruption.