fix compile error in bfs on 2.6.31

SVN-Revision: 17776

target/linux/generic-2.6/patches-2.6.31/270-sched_bfs.patch

@@ -1662,7 +1662,7 @@ This patch adds support for bfs v230, modified for diff size reduction
 +	 * combine the page table reload and the switch backend into
 +	 * one hypercall.
 +	 */
-+	arch_enter_lazy_cpu_mode();
++	arch_start_context_switch(prev);
 +
 +	if (unlikely(!mm)) {
 +		next->active_mm = oldmm;