netlink: reset extack earlier in netlink_rcv_skb

Move up the extack reset/initialization in netlink_rcv_skb, so that
those 'goto ack' will not skip it. Otherwise, later on netlink_ack
may use the uninitialized extack and cause kernel crash.

Fixes: cbbdf843 ("netlink: extack needs to be reset each time through loop")
Reported-by:  <syzbot+03bee3680a37466775e7@syzkaller.appspotmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>