Add if_inc_counter(), a generic method to update ifnet(9) counter
w/o dereferencing the struct.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271751 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f