net: ipmr/ip6mr: update lastuse on entry change

Currently lastuse is updated on entry creation and cache hit, but it should
also be updated on entry change. Since both on add and update the ttl array
is updated we can simply update the lastuse in ipmr_update_thresholds.

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