pimd: re-eval flow activity on kat expiry

pimd: re-eval flow activity on kat expiry

When the (S,G) KAT expires we need to poll for activity before dropping the
entry as traffic may have been forwarded by the dataplane since the last
periodic poll cycle.

This only works if traffic is being forwarded by the kernel i.e. if the
entries were HW accelerated via an ASIC we may still miss out on last
minute activity on the mroute in the HW.

Ticket: CM-26871

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>