ifnet/API: Privatize the implementation of the drbr_* APIs

When ALTQ is enabled ifnet accessors already need to be called, largely
defeating the purpose of the inline. To that extent, make the ALTQ form
functions in the netstack proper, and make them always available.

Reviewed By: glebius
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38104