slab: struct kmem_cache is protected by slab_mutex

cache_chain_mutex has been replaced by slab_mutex. Fix this remaining
outdated comment.
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

include/linux/slab_def.h

@@ -6,7 +6,7 @@
  */
 
 struct kmem_cache {
-/* 1) Cache tunables. Protected by cache_chain_mutex */
+/* 1) Cache tunables. Protected by slab_mutex */
 	unsigned int batchcount;
 	unsigned int limit;
 	unsigned int shared;