riscv: Use macro definition instead of magic number

The KERN_VIRT_START defines the start virtual address of kernel space.
Use this macro instead of magic number.
Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/kasan.h

@@ -13,7 +13,7 @@
 #define KASAN_SHADOW_SCALE_SHIFT	3
 
 #define KASAN_SHADOW_SIZE	(UL(1) << (38 - KASAN_SHADOW_SCALE_SHIFT))
-#define KASAN_SHADOW_START	0xffffffc000000000 /* 2^64 - 2^38 */
+#define KASAN_SHADOW_START	KERN_VIRT_START /* 2^64 - 2^38 */
 #define KASAN_SHADOW_END	(KASAN_SHADOW_START + KASAN_SHADOW_SIZE)
 
 #define KASAN_SHADOW_OFFSET	(KASAN_SHADOW_END - (1ULL << \