ppc64:Fix missing register in altivec context switch

This is a resend of a patch sent in July and that got lost somewhat, the "VSCR" register wasn't restored properly from the context on load_up_altivec (typo), please apply the fix:

===== arch/ppc64/kernel/head.S 1.76 vs edited =====

@@ -1588,6 +1588,7 @@
 	li	r10,THREAD_VSCR
 	stw	r4,THREAD_USED_VR(r5)
 	lvx	vr0,r10,r5
+	mtvscr	vr0
 	REST_32VRS(0,r4,r5)
 #ifndef CONFIG_SMP
 	/* Update last_task_used_math to 'current' */