We're not allowed to hold NET_EPOCH while sleeping, so when we call
ioctl() handlers for member interfaces we cannot be in NET_EPOCH.
We still need some protection of our CK_LISTs, so hold BRIDGE_LOCK
instead.
That requires changing BRIDGE_LOCK into a sleepable lock, and separating
the BRIDGE_RT_LOCK, to protect bridge_rtnode lists. That lock is taken
in the data path (while in NET_EPOCH), so it cannot be a sleepable lock.
While here document the locking stategy.