powerpc/kasan: Fix issues by lowering KASAN_SHADOW_END

At the time being, KASAN_SHADOW_END is 0x100000000, which
is 0 in 32 bits representation.

This leads to a couple of issues:
- kasan_remap_early_shadow_ro() does nothing because the comparison
k_cur < k_end is always false.
- In ptdump, address comparison for markers display fails and the
marker's name is printed at the start of the KASAN area instead of
being printed at the end.

However, there is no need to shadow the KASAN shadow area itself,
so the KASAN shadow area can stop shadowing memory at the start
of itself.

With a PAGE_OFFSET set to 0xc0000000, KASAN shadow area is then going
from 0xf8000000 to 0xff000000.

Fixes: cbd18991 ("powerpc/mm: Fix an Oops in kasan_mmu_init()")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ae1a3c0d19a37410c209c3fc453634cfcc0ee318.1589866984.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/kasan.h

@@ -23,9 +23,7 @@
 
 #define KASAN_SHADOW_OFFSET	ASM_CONST(CONFIG_KASAN_SHADOW_OFFSET)
 
-#define KASAN_SHADOW_END	0UL
-
-#define KASAN_SHADOW_SIZE	(KASAN_SHADOW_END - KASAN_SHADOW_START)
+#define KASAN_SHADOW_END	(-(-KASAN_SHADOW_START >> KASAN_SHADOW_SCALE_SHIFT))
 
 #ifdef CONFIG_KASAN
 void kasan_early_init(void);