SUNRPC: rpc_timeout_upcall_queue should not sleep

 The function rpc_timeout_upcall_queue runs from a workqueue, and hence
 sleeping is not recommended. Convert the protection of the upcall queue
 from being mutex-based to being spinlock-based.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>