powerpc/8xx: set STACK_END_MAGIC earlier on the init_stack

Today, the STACK_END_MAGIC is set on init_stack in start_kernel().

To avoid a false 'Thread overran stack, or stack corrupted' message
on early Oopses, setup STACK_END_MAGIC as soon as possible.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/54f67bb7ac486c1350f2fa8905cd279f94b9dfb1.1566382841.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/head_8xx.S

@@ -15,6 +15,7 @@
  */
 
 #include <linux/init.h>
+#include <linux/magic.h>
 #include <asm/processor.h>
 #include <asm/page.h>
 #include <asm/mmu.h>
@@ -717,6 +718,9 @@ start_here:
 	/* stack */
 	lis	r1,init_thread_union@ha
 	addi	r1,r1,init_thread_union@l
+	lis	r0, STACK_END_MAGIC@h
+	ori	r0, r0, STACK_END_MAGIC@l
+	stw	r0, 0(r1)
 	li	r0,0
 	stwu	r0,THREAD_SIZE-STACK_FRAME_OVERHEAD(r1)