netgraph/ng_bridge: Reestablish old ABI

In order to be able to merge r353026 bring back support for the old
cookie API for a transition period in 12.x releases (and possibly 13)
before the old API can be removed again entirely.

Suggested by: julian
Submitted by: Lutz Donnerhacke (lutz donnerhacke.de)
PR: 240787
Reviewed by: julian
MFC after: 2 weeks
X-MFC with: r353026
Differential Revision: https://reviews.freebsd.org/D21961