Once we've sent nd6_mmaxtries NS messages, send RTM_MISS and move to the
ND6_LLINFO_WAITDELETE state rather than freeing the llentry right away.
Wait for a probe cycle and then free the llentry.
If a connection attempts to re-use the llentry during ND6_LLINFO_WAITDELETE,
return EHOSTDOWN (or EHOSTUNREACH if a gateway) to match inet behaviour.
Continue to ND6_LLINFO_INCOMPLETE and send another NS probe in hope of a
reply. Rinse and repeat.
This reverts part of nd6.c r1.14 - an 18 year old commit!
ca7497cb84