netgraph/ng_bridge: switch stats to counter framework

This is the first patch of a series of necessary steps
to make ng_bridge(4) multithreaded.

Reviewed by: melifaro (network), afedorov
Differential Revision: https://reviews.freebsd.org/D28125

(cherry picked from commit 66c72859f66dc6c852234589f3508ce5d36d0336)