sunrpc: only free unix grouplist after RCU settles

While the unix_gid object is rcu-freed, the group_info list that it
contains is not. Ensure that we only put the group list reference once
we are really freeing the unix_gid object.
Reported-by: Zhi Li <yieli@redhat.com>
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2183056Signed-off-by: Jeff Layton <jlayton@kernel.org>
Fixes: fd5d2f78 ("SUNRPC: Make server side AUTH_UNIX use lockless lookups")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svcauth_unix.c

@@ -416,14 +416,23 @@ static int unix_gid_hash(kuid_t uid)
 	return hash_long(from_kuid(&init_user_ns, uid), GID_HASHBITS);
 }
 
-static void unix_gid_put(struct kref *kref)
+static void unix_gid_free(struct rcu_head *rcu)
 {
-	struct cache_head *item = container_of(kref, struct cache_head, ref);
-	struct unix_gid *ug = container_of(item, struct unix_gid, h);
+	struct unix_gid *ug = container_of(rcu, struct unix_gid, rcu);
+	struct cache_head *item = &ug->h;
+
 	if (test_bit(CACHE_VALID, &item->flags) &&
 	    !test_bit(CACHE_NEGATIVE, &item->flags))
 		put_group_info(ug->gi);
-	kfree_rcu(ug, rcu);
+	kfree(ug);
+}
+
+static void unix_gid_put(struct kref *kref)
+{
+	struct cache_head *item = container_of(kref, struct cache_head, ref);
+	struct unix_gid *ug = container_of(item, struct unix_gid, h);
+
+	call_rcu(&ug->rcu, unix_gid_free);
 }
 
 static int unix_gid_match(struct cache_head *corig, struct cache_head *cnew)