bridge: store a bridge_iflist pointer in ifnet

currently, bridge stores a pointer to its softc in ifnet. this means
that when processing a packet, it needs to walk the bridge's member list
to find the bridge_iflist for the interface the packet came from.

instead, store a pointer to the bridge_iflist in ifnet, and add a
pointer from bridge_iflist back to the bridge_softc. this means given
an ifnet, we always have both the softc and the bridge_iflist without a
list walk.

there are two places outside if_bridge that treat ifnet->if_bridge as
something other than an opaque pointer: bridgestp, and netinet. add two
function pointers exported from if_bridge to handle those cases.

bump __FreeBSD_version as this is technically a KABI break.

Reviewed by: kp