pimd: Ensure new generation_id is different from previous

pimd: Ensure new generation_id is different from previous

The RFC states that an interfaces generation_id must be changed
if it experiences an if down.  From 4.3.1:

The GenID option contains a randomly generated
32-bit value that is regenerated each time PIM forwarding is started
or restarted on the interface, including when the router itself
restarts.

Since we are only grabbing a new generation_id without comparing
it to the previous generation_id, it is possible that random
can generate the exact same number.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>