kasan: turn on -fsanitize-address-use-after-scope

In the upcoming gcc7 release, the -fsanitize=kernel-address option at
first implied new -fsanitize-address-use-after-scope option.  This would
cause link errors on older kernels because they don't have two new
functions required for use-after-scope support.  Therefore, gcc7 changed
default to -fno-sanitize-address-use-after-scope.

Now the kernel has everything required for that feature since commit
828347f8f9 ("kasan: support use-after-scope detection").  So, to make it
work, we just have to enable use-after-scope in CFLAGS.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrey Ryabinin <[email protected]>
Acked-by: Dmitry Vyukov <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

scripts/Makefile.kasan

@@ -28,4 +28,6 @@ else
         CFLAGS_KASAN := $(CFLAGS_KASAN_MINIMAL)
     endif
 endif
+
+CFLAGS_KASAN += $(call cc-option, -fsanitize-address-use-after-scope)
 endif