xtensa: fix build with KASAN

xtensa: fix build with KASAN

The commit 917538e212a2 ("kasan: clean up KASAN_SHADOW_SCALE_SHIFT
usage") removed KASAN_SHADOW_SCALE_SHIFT definition from
include/linux/kasan.h and added it to architecture-specific headers,
except for xtensa. This broke the xtensa build with KASAN enabled.
Define KASAN_SHADOW_SCALE_SHIFT in arch/xtensa/include/asm/kasan.h

Reported by: kbuild test robot <[email protected]>
Fixes: 917538e212a2 ("kasan: clean up KASAN_SHADOW_SCALE_SHIFT usage")
Acked-by: Andrey Konovalov <[email protected]>
Signed-off-by: Max Filippov <[email protected]>