vrouter: fixing incorrect length values in packet headers

There has been mistake made during calculation of values for
size attributes of headers structures in Netlink protocol.
Mostly the size of header has been omitted, while should be
included.
Additionally comments in netlink.h has been updated to better
explain the meaning of length fields in Nelink header structures.
Merge pull request #1 from Semihalf/freebsd__vrouter_netlink_fix

vrouter: fixing incorrect length values in packet headers