Add a function to set if_get_counter method for an ifnet. To be used
in the drivers that are already converted to "Juniper drvapi". This
can be revisited in future.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271770 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f