ipmr, ip6mr: add RTNH_F_UNRESOLVED flag to unresolved cache entries

While working with ipmr, we noticed that it is impossible to determine
if an entry is actually unresolved or its IIF interface has disappeared
(e.g. virtual interface got deleted). These entries look almost
identical to user-space when dumping or receiving notifications. So in
order to recognize them add a new RTNH_F_UNRESOLVED flag which is set when
sending an unresolved cache entry to user-space.

Suggested-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>