xfrm: Fix xfrm_policy_lock imbalance

An earlier patch accidentally replaced a write_lock_bh
with a spin_unlock_bh. Fix this by using spin_lock_bh
instead.

Fixes: 9d0380df ("xfrm: policy: convert policy_lock to spinlock")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -979,7 +979,7 @@ int xfrm_policy_flush(struct net *net, u8 type, bool task_valid)
 
 			xfrm_policy_kill(pol);
 
-			spin_unlock_bh(&net->xfrm.xfrm_policy_lock);
+			spin_lock_bh(&net->xfrm.xfrm_policy_lock);
 			goto again1;
 		}