kasan: fix variable 'tag' set but not used warning

set_tag() compiles away when CONFIG_KASAN_SW_TAGS=n, so make
arch_kasan_set_tag() a static inline function to fix warnings below.

  mm/kasan/common.c: In function '__kasan_kmalloc':
  mm/kasan/common.c:475:5: warning: variable 'tag' set but not used [-Wunused-but-set-variable]
    u8 tag;
       ^~~

Link: http://lkml.kernel.org/r/20190307185244.54648-1-cai@lca.pw
Signed-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kasan/kasan.h

@@ -163,7 +163,10 @@ static inline u8 random_tag(void)
 #endif
 
 #ifndef arch_kasan_set_tag
-#define arch_kasan_set_tag(addr, tag)	((void *)(addr))
+static inline const void *arch_kasan_set_tag(const void *addr, u8 tag)
+{
+	return addr;
+}
 #endif
 #ifndef arch_kasan_reset_tag
 #define arch_kasan_reset_tag(addr)	((void *)(addr))