SUNRPC: prevent task_cleanup running on freed xprt

We saw a report of a NULL dereference in xprt_autoclose:

	https://bugzilla.redhat.com/show_bug.cgi?id=611938

This appears to be the result of an xprt's task_cleanup running after
the xprt is destroyed.  Nothing in the current code appears to prevent
that.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

net/sunrpc/xprt.c

@@ -1131,6 +1131,7 @@ static void xprt_destroy(struct kref *kref)
 	rpc_destroy_wait_queue(&xprt->sending);
 	rpc_destroy_wait_queue(&xprt->resend);
 	rpc_destroy_wait_queue(&xprt->backlog);
+	cancel_work_sync(&xprt->task_cleanup);
 	/*
 	 * Tear down transport state and free the rpc_xprt
 	 */