mm, slab: periodically resched in drain_freelist()

drain_freelist() can be called with a very large number of slabs to free,
such as for kmem_cache_shrink(), or depending on various settings of the
slab cache when doing periodic reaping.

If there is a potentially long list of slabs to drain, periodically
schedule to ensure we aren't saturating the cpu for too long.
Signed-off-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slab.c

@@ -2211,6 +2211,8 @@ static int drain_freelist(struct kmem_cache *cache,
 		raw_spin_unlock_irq(&n->list_lock);
 		slab_destroy(cache, slab);
 		nr_freed++;
+
+		cond_resched();
 	}
 out:
 	return nr_freed;