SUNRPC: Prevent looping due to rpc_signal_task() races

If rpc_signal_task() is called while a task is in an rpc_call_done()
callback function, and the latter calls rpc_restart_call(), the task can
end up looping due to the RPC_TASK_SIGNALLED flag being set without the
tk_rpc_status being set.
Removing the redundant mechanism for signalling the task fixes the
looping behaviour.

Reported-by: Li Lingfeng <lilingfeng3@huawei.com>
Fixes: 39494194 ("SUNRPC: Fix races with rpc_killall_tasks()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>