powerpc: Fix bug in move of altivec code to vector.S

The patch that moved to vector.S and made common between 32 and 64-bit the
altivec code had a nasty bug on 32-bit (did I really test that ?) which
causes the kernel to blr back into userspace ... oops :-)
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/head_32.S

@@ -733,9 +733,11 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_NEED_DTLB_SW_LRU)
 AltiVecUnavailable:
 	EXCEPTION_PROLOG
 #ifdef CONFIG_ALTIVEC
-	bne	load_up_altivec		/* if from user, just load it up */
+	beq	1f
+	bl	load_up_altivec		/* if from user, just load it up */
+	b	fast_exception_return
 #endif /* CONFIG_ALTIVEC */
-	addi	r3,r1,STACK_FRAME_OVERHEAD
+1:	addi	r3,r1,STACK_FRAME_OVERHEAD
 	EXC_XFER_EE_LITE(0xf20, altivec_unavailable_exception)
 
 PerformanceMonitor: