netlink: fix `ipaddr` NLA type
netlink: fix `ipaddr` NLA type

Don't assume the ipaddr family by the message family field. Instead, use
the value length (to decode) or a colon (:) (to encode) to get the family.

Bug-Url: https://github.com/svinota/pyroute2/issues/151
little-dude/pyroute2,nazarewk/pyroute2,simudream/pyroute2,drzaeus77/pyroute2,tomislacker/python-iproute2,vodik/pyroute2,vodik/pyroute2,drzaeus77/pyroute2,roolebo/pyroute2,tomislacker/python-iproute2,little-dude/pyroute2,mtiny/pyroute2,simudream/pyroute2,nazarewk/pyroute2,mtiny/pyroute2,nazarewk/pyroute2,roolebo/pyroute2,little-dude/pyroute2