flowcache: Avoid OOM condition under preasure

We can hit an OOM condition if we are under presure because
we can not free the entries in gc_list fast enough. So add
a counter for the not yet freed entries in the gc_list and
refuse new allocations if the value is too high.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/include/net/netns/xfrm.h b/include/net/netns/xfrm.h
index 730d82a..24cd394 100644
--- a/include/net/netns/xfrm.h
+++ b/include/net/netns/xfrm.h
@@ -80,6 +80,7 @@
 	struct flow_cache	flow_cache_global;
 	atomic_t		flow_cache_genid;
 	struct list_head	flow_cache_gc_list;
+	atomic_t		flow_cache_gc_count;
 	spinlock_t		flow_cache_gc_lock;
 	struct work_struct	flow_cache_gc_work;
 	struct work_struct	flow_cache_flush_work;