samples: net: echo-server: Add netmask setting to VLAN config

We need to set a proper IPv4 netmask to each VLAN interface
so that IPv4 source address selection will work properly.
If we do not do this, then the network interface could be
selected incorrectly for the VLAN interfaces which could then
mean that the packet might get dropped by the target device.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>