s390/head: avoid doubling early boot stack size under KASAN

Early boot stack uses predefined 4 pages of memory 0x8000-0xC000. This
stack is used to run not instumented decompressor/facilities
verification C code. It doesn't make sense to double its size when
the kernel is built with KASAN support. BOOT_STACK_ORDER is introduced
to avoid that.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/head.S

@@ -315,7 +315,7 @@ ENTRY(startup_kdump)
 	brasl	%r14,startup_kernel
 
 .Lstack:
-	.long	0x8000 + THREAD_SIZE - STACK_FRAME_OVERHEAD
+	.long	0x8000 + (1<<(PAGE_SHIFT+BOOT_STACK_ORDER)) - STACK_FRAME_OVERHEAD
 	.align	8
 6:	.long	0x7fffffff,0xffffffff