mm/slub.c: fix -Wunused-function compiler warnings

tid_to_cpu() and tid_to_event() are only used in note_cmpxchg_failure()
when SLUB_DEBUG_CMPXCHG=y, so when SLUB_DEBUG_CMPXCHG=n by default, Clang
will complain that those unused functions.

Link: http://lkml.kernel.org/r/1568752232-5094-1-git-send-email-cai@lca.pwSigned-off-by: Qian Cai <cai@lca.pw>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -2004,6 +2004,7 @@ static inline unsigned long next_tid(unsigned long tid)
 	return tid + TID_STEP;
 }
 
+#ifdef SLUB_DEBUG_CMPXCHG
 static inline unsigned int tid_to_cpu(unsigned long tid)
 {
 	return tid % TID_STEP;
@@ -2013,6 +2014,7 @@ static inline unsigned long tid_to_event(unsigned long tid)
 {
 	return tid / TID_STEP;
 }
+#endif
 
 static inline unsigned int init_tid(int cpu)
 {