network: radv: Send RA on newly-added dynamic prefix

When a prefix is delegated to an interface that is already sending
RAs, send an RA immediately to inform clients of the new prefix.
This allows them to start using it immediately instead of waiting
up to nearly 10 minutes (depending on when the last timed RA was
sent).  This type of situation might occur if, for example, an
outage of the WAN connection caused the addresses and prefixes to
be lost and later regained after service was restored.  The
condition for the number of RAs sent being above 0 simultaneously
ensures that RADV is already running and that this code doesn't
send any RAs before the timed RAs have started when the interface
first comes up.