netpoll: netpoll_send_skb() returns transmit status

[ Upstream commit 1ddabdfaf70c202b88925edd74c66f4707dbd92e ]

Some callers want to know if the packet has been sent or
dropped, to inform upper stacks.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 505ead7ab77f ("netpoll: hold rcu read lock in __netpoll_send_skb()")
Signed-off-by: Sasha Levin <sashal@kernel.org>