BSD: Fix netmask family and length for incoming route msgs

BSD: Fix netmask family and length for incoming route msgs

Netmask family and length are ignored by traditional
userland tools such as route and netstat and are assumed
to match the destination sockaddr.
This is fortunate because BSD kernels use a radix tree
to store routes which adjusts the netmask at the point
of insertion where this information is lost.
We can just sub in the values from the destination address.

This is currently true for all BSD kernels.