kasan/arm64: fix CONFIG_KASAN_SW_TAGS && KASAN_INLINE

The generic Makefile.kasan propagates CONFIG_KASAN_SHADOW_OFFSET into
KASAN_SHADOW_OFFSET, but only does so for CONFIG_KASAN_GENERIC.

Since commit:

  6bd1d0be ("arm64: kasan: Switch to using KASAN_SHADOW_OFFSET")

... arm64 defines CONFIG_KASAN_SHADOW_OFFSET in Kconfig rather than
defining KASAN_SHADOW_OFFSET in a Makefile. Thus, if
CONFIG_KASAN_SW_TAGS && KASAN_INLINE are selected, we get build time
splats due to KASAN_SHADOW_OFFSET not being set:

| [mark@lakrids:~/src/linux]% usellvm 8.0.1 usekorg 8.1.0  make ARCH=arm64 CROSS_COMPILE=aarch64-linux- CC=clang
| scripts/kconfig/conf  --syncconfig Kconfig
|   CC      scripts/mod/empty.o
| clang (LLVM option parsing): for the -hwasan-mapping-offset option: '' value invalid for uint argument!
| scripts/Makefile.build:273: recipe for target 'scripts/mod/empty.o' failed
| make[1]: *** [scripts/mod/empty.o] Error 1
| Makefile:1123: recipe for target 'prepare0' failed
| make: *** [prepare0] Error 2

Let's fix this by always propagating CONFIG_KASAN_SHADOW_OFFSET into
KASAN_SHADOW_OFFSET if CONFIG_KASAN is selected, moving the existing
common definition of +CFLAGS_KASAN_NOSANITIZE to the top of
Makefile.kasan.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Tested-by Steve Capper <steve.capper@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

scripts/Makefile.kasan

 # SPDX-License-Identifier: GPL-2.0
+ifdef CONFIG_KASAN
+CFLAGS_KASAN_NOSANITIZE := -fno-builtin
+KASAN_SHADOW_OFFSET ?= $(CONFIG_KASAN_SHADOW_OFFSET)
+endif
+
 ifdef CONFIG_KASAN_GENERIC
 
 ifdef CONFIG_KASAN_INLINE
@@ -7,8 +12,6 @@ else
 	call_threshold := 0
 endif
 
-KASAN_SHADOW_OFFSET ?= $(CONFIG_KASAN_SHADOW_OFFSET)
-
 CFLAGS_KASAN_MINIMAL := -fsanitize=kernel-address
 
 cc-param = $(call cc-option, -mllvm -$(1), $(call cc-option, --param $(1)))
@@ -45,7 +48,3 @@ CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
 		$(instrumentation_flags)
 
 endif # CONFIG_KASAN_SW_TAGS
-
-ifdef CONFIG_KASAN
-CFLAGS_KASAN_NOSANITIZE := -fno-builtin
-endif