Fix MTU as soon as possible.

If a probe reply is received that makes minmtu equal to maxmtu, we
have to wait until try_mtu() runs to realize that. Since try_mtu()
runs after a packet is sent, this means there is at least one packet
(possibly more, depending on timing) that won't benefit from the
fixed MTU. This also happens when maxmtu is updated from the send()
path.

This commit fixes that by making sure we check whether the MTU can be
fixed every time minmtu or maxmtu is touched.