When configuring a new address on an interface, an upstream router

When configuring a new address on an interface, an upstream router
doesn't know where to send traffic.  This will send an unsolicited
neighbor advertisement, as described in RFC9131, to the all-routers
multicast address so all routers on the same link will learn the path
back to the address.

This is intended to speed up the first return packet on an IPv6 interface.

OK florian@