ipmr, ip6mr: return lastuse relative to now

When I introduced the lastuse member I made a subtle error because it was
returned as an absolute value but that is meaningless to user-space as it
doesn't allow to see how old exactly an entry is. Let's make it similar to
how the bridge returns such values and make it relative to "now" (jiffies).
This allows us to show the actual age of the entries and is much more
useful (e.g. user-space daemons can age out entries, iproute2 can display
the lastuse properly).

Fixes: 43b9e127 ("net: ipmr/ip6mr: add support for keeping an entry age")
Reported-by: Satish Ashok <sashok@cumulusnetworks.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>