riscv: Fix asan-stack clang build

Nathan reported that because KASAN_SHADOW_OFFSET was not defined in
Kconfig, it prevents asan-stack from getting disabled with clang even
when CONFIG_KASAN_STACK is disabled: fix this by defining the
corresponding config.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Fixes: 8ad8b72721d0 ("riscv: Add KASAN support")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index c3f3fd5..6d5b63b 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -163,6 +163,12 @@
 	default 0xffffffff80000000 if 64BIT && MAXPHYSMEM_2GB
 	default 0xffffffe000000000 if 64BIT && MAXPHYSMEM_128GB
 
+config KASAN_SHADOW_OFFSET
+	hex
+	depends on KASAN_GENERIC
+	default 0xdfffffc800000000 if 64BIT
+	default 0xffffffff if 32BIT
+
 config ARCH_FLATMEM_ENABLE
 	def_bool !NUMA