ppc64: Fix booting on LPAR machines with more than 1 CPU

Hi !

The exception rewrite contains a small bug that prevents bring up of CPUs on logically partitioned machines. The kernel is trying to zero the backlink on the new stack while running with relocation disabled, which potentially cause it to try to access an address outside of the region allowed in real mode. This seem to be a leftover from previous code as we also zero the backlink later after turning off the MMU. This patch removes the offending bit.

===== arch/ppc64/kernel/head.S 1.61 vs edited =====

@@ -1833,8 +1833,6 @@
 	sldi	r28,r24,3		/* get current_set[cpu#] */
 	ldx	r1,r3,r28
 	addi	r1,r1,THREAD_SIZE-STACK_FRAME_OVERHEAD
-	li	r0,0
-	std	r0,0(r1)
 	std	r1,PACAKSAVE(r13)
 
 	ld	r3,PACASTABREAL(r13)    /* get raddr of segment table       */