nfsd: only check RPC_SIGNALLED() when restarting rpc_task

nfsd: only check RPC_SIGNALLED() when restarting rpc_task

nfsd4_cb_sequence_done() currently checks RPC_SIGNALLED() when
processing the compound and releasing the slot. If RPC_SIGNALLED()
returns true, then that means that the client is going to be torn down.

Don't check RPC_SIGNALLED() after processing a successful reply. Check
it only before restarting the rpc_task. If it returns true, then requeue
the callback instead of restarting the task.

Also, handle rpc_restart_call() and rpc_restart_call_prepare() failures
correctly, by requeueing the callback.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>