netgraph/ng_bridge: Make simple internal functions read-only

The data path in netgraph is designed to work on an read only state of
the whole netgraph network. Currently this is achived by convetion,
there is no technical enforcment. This patch is part of a series to
make ng_brigde multithreaded, which is done by rewrite the data path
to const handling.

Reviewed By: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28141