xtensa: fix build with KASAN

The commit 917538e2 ("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 <fengguang.wu@intel.com>
Fixes: 917538e2 ("kasan: clean up KASAN_SHADOW_SCALE_SHIFT usage")
Acked-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/kasan.h

@@ -10,6 +10,8 @@
 #include <linux/sizes.h>
 #include <asm/kmem_layout.h>
 
+#define KASAN_SHADOW_SCALE_SHIFT 3
+
 /* Start of area covered by KASAN */
 #define KASAN_START_VADDR __XTENSA_UL_CONST(0x90000000)
 /* Start of the shadow map */