ospfd: don't allow to set network type on loopback interfaces

ospfd: don't allow to set network type on loopback interfaces

OSPFd only allocates some stub information for loopback interfaces.
This causes a crash when the interface state machine is started on
that interface by configuring a different network type.

It doesn't make much sense to configure the network type of a loopback
interface, therefore, just forbid it.

See also bugzilla #670.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>