bridge: do not allow a bridge SVI in a bridge

Disallow this:

ifconfig bridge0 create
ifconfig bridge0.1 create
ifconfig bridge0 addm bridge0.1

Also disallow this:

ifconfig vlan1 create
ifconfig bridge0 create
ifconfig bridge0 addm vlan1
ifconfig vlan1 vlan 1 vlandev bridge0

Firstly, this panics due to trying to take BRIDGE_LOCK recursively.
Secondly, even if it worked, it could cause packet forwarding loops.

Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D51310