ifconfig_bxe0_ipv6="inet6 accept_rtadv" seems to break bridging

Hi,

context is 15.0-CURRENT #0 n278914-e0838f8a2e61 
GENERIC-NODEBUG amd64 1500053 1500053

I've found ifconfig_bxe0_ipv6="inet6 accept_rtadv" breaks bridging
on a dual stack interface in unless some manual hoops are jumped through.

net.link.bridge.member_ifaddrs is the default ("0") here.

Imagine that 203.0.113.0/24 is real and routable.

The following works as expected. if the machine reboots,
ipv4 starts normally.

cloned_interfaces="bridge0 tap0"
ifconfig_bxe0="up"
ifconfig_bridge0="203.0.113.2/24 addm bxe0 addm tap0"
defaultrouter="203.0.113.1"

Adding ifconfig_bxe0_ipv6="inet6 accept_rtadv" and then rebooting causes the
following:

Initially, only ipv6 works. ipv4 shows the following responses to route 
commands:

# route show 1.1.1.1
route: route has not been found

# route add default 203.0.113.1
route: message indicates error: Invalid argument
add net default: gateway 203.0.113.1 fib 0: Invalid argument

Running service netif restart stops ipv6 routing and ipv4 starts working
normally. In order to have functioning dual stack, I have to
'sysctl net.link.bridge.member_ifaddrs=1' and netif restart.

Is this known/expected?
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