netpoll: accept NULL np argument in netpoll_send_skb()

mainline inclusion
from mainline-v5.11-rc2
commit f78ed220
category: bugfix
bugzilla: 46921
CVE: NA

-------------------

netpoll_send_skb() callers seem to leak skb if
the np pointer is NULL. While this should not happen, we
can make the code more robust.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Aichun Li <liaichun@huawei.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: wangxiaopeng <wangxiaopeng7@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>