wireguard: netlink: send staged packets when setting initial private key

Packets bound for peers can queue up prior to the device private key
being set. For example, if persistent keepalive is set, a packet is
queued up to be sent as soon as the device comes up. However, if the
private key hasn't been set yet, the handshake message never sends, and
no timer is armed to retry, since that would be pointless.

But, if a user later sets a private key, the expectation is that those
queued packets, such as a persistent keepalive, are actually sent. So
adjust the configuration logic to account for this edge case, and add a
test case to make sure this works.

Maxim noticed this with a wg-quick(8) config to the tune of:

    [Interface]
    PostUp = wg set %i private-key somefile

    [Peer]
    PublicKey = ...
    Endpoint = ...
    PersistentKeepalive = 25

Here, the private key gets set after the device comes up using a PostUp
script, triggering the bug.

Fixes: e7096c13

 ("net: WireGuard secure network tunnel")
Cc: stable@vger.kernel.org
Reported-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Tested-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Link: https://lore.kernel.org/wireguard/87fs7xtqrv.fsf@gmail.com/


Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>