ipmroute: add support for age dumping

Add support to dump the mroute cache entry age if the show_stats (-s)
switch is provided.
Example:
$ ip -s mroute
(0.0.0.0, 239.10.10.10)          Iif: eth0       Oifs: eth0
  0 packets, 0 bytes, Age  245.44

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
1 file changed