SUNRPC: The function rpc_restart_call() should return success/failure

Both rpc_restart_call_prepare() and rpc_restart_call() test for the
RPC_TASK_KILLED flag, and fail to restart the RPC call if that flag is set.

This patch allows callers to know whether or not the restart was
successful, so that they can perform cleanups etc in case of failure.

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