Check return value from reading name_assign_type attr

This file won't exist on kernels earlier than 3.17.
udev: link-config - simplify net-match
networkd: route - ignore unknown address family
sd-rtnl: fix bogus warning about dropping 20 bytes from multi-part messages

Nothing was being dropped, we just failed to account for the NLMSG_DONE.
networkd: link - typo