It turns out there's an easy way to get packets queued up while still
having an MTU of zero, and that's via persistent keep alive. This commit
makes sure that in whatever condition, we don't wind up dividing by
zero. Note that an MTU of zero for a wireguard interface is something
quasi-valid, so I don't think the correct fix is to limit it via
min_mtu. This can be reproduced easily with:

ip link add wg0 type wireguard
ip link add wg1 type wireguard
ip link set wg0 up mtu 0
ip link set wg1 up
wg set wg0 private-key <(wg genkey)
wg set wg1 listen-port 1 private-key <(wg genkey) peer $(wg show wg0 public-key)
wg set wg0 peer $(wg show wg1 public-key) persistent-keepalive 1 endpoint 127.0.0.1:1

However, while min_mtu=0 seems fine, it makes sense to restrict the
max_mtu. This commit also restricts the maximum MTU to the greatest
number for which rounding up to the padding multiple won't overflow a
signed integer. Packets this large were always rejected anyway
eventually, due to checks deeper in, but it seems more sound not to even
let the administrator configure something that won't work anyway.

We use this opportunity to clean up this function a bit so that it's
clear which paths we're expecting.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>