powerpc: Initialise paca->kstack before early_setup_secondary

As early setup calls down to slb_initialize(), we must have kstack
initialised before checking "should we add a bolted SLB entry for our kstack?"

Failing to do so means stack access requires an SLB miss exception to refill
an entry dynamically, if the stack isn't accessible via SLB(0) (kernel text
& static data).  It's not always allowable to take such a miss, and
intermittent crashes will result.

Primary CPUs don't have this issue; an SLB entry is not bolted for their
stack anyway (as that lives within SLB(0)).  This patch therefore only
affects the init of secondaries.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/head_64.S

@@ -572,16 +572,16 @@ __secondary_start:
 	/* Set thread priority to MEDIUM */
 	HMT_MEDIUM
 
-	/* Do early setup for that CPU (stab, slb, hash table pointer) */
-	bl	.early_setup_secondary
-
 	/* Initialize the kernel stack.  Just a repeat for iSeries.	 */
 	LOAD_REG_ADDR(r3, current_set)
 	sldi	r28,r24,3		/* get current_set[cpu#]	 */
 	ldx	r1,r3,r28
 	addi	r1,r1,THREAD_SIZE-STACK_FRAME_OVERHEAD
 	std	r1,PACAKSAVE(r13)
 
+	/* Do early setup for that CPU (stab, slb, hash table pointer) */
+	bl	.early_setup_secondary
+
 	/* Clear backchain so we get nice backtraces */
 	li	r7,0
 	mtlr	r7