network: set mtu of unpriv veth to the bridge's mtu

That's to make sure that if the bridge has a higher than 1500
mtu, the new veth (defaulting to 1500) doesn't lower it.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>