bridge: Release bridge lock when calling iflladdr_event handler

[kp]

Summary

We can't run the event handler with the bridge lock held because it can
trigger arp packets on the bridge, which would try to take tbe bridge
lock again.
We do ensure that no interfaces can be added or removed while we do
this.

The crash manifests when adding/removing the first interface to the
bridge if net.link.bridge.inherit_mac is set.

PR: 216510

Test Plan

bridge=ifconfig bridge create
epair=ifconfig epair create | sed 's/.$//'

sysctl net.link.bridge.inherit_mac=1
ifconfig $bridge 11.255.255.1/28
ifconfig $bridge addm ${epair}a
ifconfig $bridge deletem ${epair}a