bridge: Change lists to CK_LIST as a peparation for epochification

Prepare the ground for a rework of the bridge locking approach. We will
use an epoch-based approach in the datapath and making it safe to
iterate over the interface, span and rtnode lists without holding the
BRIDGE_LOCK. Replace the relevant lists by their ConcurrencyKit
equivalents.

No functional change in this commit.

Reviewed by: emaste, ae, philip (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24249