xfrm: don't call xfrm_policy_cache_flush while holding spinlock

xfrm_policy_cache_flush can sleep, so it cannot be called while holding
a spinlock.  We could release the lock first, but I don't see why we need
to invoke this function here in first place, the packet path won't reuse
an xdst entry unless its still valid.

While at it, add an annotation to xfrm_policy_cache_flush, it would
have probably caught this bug sooner.

Fixes: ec30d78c14a813 ("xfrm: add xdst pcpu cache")
Reported-by: syzbot+e149f7d1328c26f9c12f@syzkaller.appspotmail.com
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -975,8 +975,6 @@ int xfrm_policy_flush(struct net *net, u8 type, bool task_valid)
 	}
 	if (!cnt)
 		err = -ESRCH;
-	else
-		xfrm_policy_cache_flush();
 out:
 	spin_unlock_bh(&net->xfrm.xfrm_policy_lock);
 	return err;
@@ -1744,6 +1742,8 @@ void xfrm_policy_cache_flush(void)
 	bool found = 0;
 	int cpu;
 
+	might_sleep();
+
 	local_bh_disable();
 	rcu_read_lock();
 	for_each_possible_cpu(cpu) {