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 convention,
there is no technical enforcment. In the case of NETGRAPH_DEBUG all
nodes can be annotated for debugging purposes, so the strict
enforcment needs to be lifted for this purpose.

This patch is part of a series to make ng_bridge multithreaded, which
is done by rewrite the data path to operate on const.

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