export: Skip writing any subnets

export: Skip writing any subnets

It is unnecessary to write any subnets in the output, because
they might already be included in a bigger parent network.

Since the tree is ordered, we only need to check if the network
to be written is a subnet of the previous one. If so, we skip it.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>