vmscan: use atomic-long for shrinker batching

Use atomic-long operations instead of looping around cmpxchg().

[akpm@linux-foundation.org: massage atomic.h inclusions]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/shrinker.h

@@ -35,7 +35,7 @@ struct shrinker {
 
 	/* These are for internal use */
 	struct list_head list;
-	long nr;	/* objs pending delete */
+	atomic_long_t nr_in_batch; /* objs pending delete */
 };
 #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
 extern void register_shrinker(struct shrinker *);