powerpc/kasan: Fix KASAN_SHADOW_START on BOOK3S_32

On BOOK3S_32, when we have modules and strict kernel RWX, modules
are not in vmalloc space but in a dedicated segment that is
below PAGE_OFFSET.

So KASAN_SHADOW_START must take it into account.

MODULES_VADDR can't be used because it is not defined yet
in kasan.h

Fixes: 6ca05532 ("powerpc/32s: Use dedicated segment for modules with STRICT_KERNEL_RWX")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6eddca2d5611fd57312a88eae31278c87a8fc99d.1596641224.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/kasan.h

@@ -15,11 +15,18 @@
 #ifndef __ASSEMBLY__
 
 #include <asm/page.h>
+#include <linux/sizes.h>
 
 #define KASAN_SHADOW_SCALE_SHIFT	3
 
+#if defined(CONFIG_PPC_BOOK3S_32) && defined(CONFIG_MODULES) && defined(CONFIG_STRICT_KERNEL_RWX)
+#define KASAN_KERN_START	ALIGN_DOWN(PAGE_OFFSET - SZ_256M, SZ_256M)
+#else
+#define KASAN_KERN_START	PAGE_OFFSET
+#endif
+
 #define KASAN_SHADOW_START	(KASAN_SHADOW_OFFSET + \
-				 (PAGE_OFFSET >> KASAN_SHADOW_SCALE_SHIFT))
+				 (KASAN_KERN_START >> KASAN_SHADOW_SCALE_SHIFT))
 
 #define KASAN_SHADOW_OFFSET	ASM_CONST(CONFIG_KASAN_SHADOW_OFFSET)