netgraph/ng_bridge: Merge internal structures

In a earlier version of ng_bridge(4) the exernal visible host entry
structure was a strict subset of the internal one. So internal view
was a direct annotation of the external structure. This strict
inheritance was lost many versions ago. There is no need to
encapsulate a part of the internal represntation as a separate
structure.

This patch is a preparation to make the internal structure read only
in the data path in order to make ng_bridge(4) multithreaded.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D28545

(cherry picked from commit ccf4cd2e7830394467d5f6cf546ab453f9657b69)