zebra: Fix dplane_fpm_nl to allow for fast configuration

zebra: Fix dplane_fpm_nl to allow for fast configuration

If you have this order in your configuration file:

no fpm use-next-hop-groups
fpm address 127.0.0.1

the dplane code was using the same event thread t_event and the second
add event in the code was going, you already have an event scheduled
and as such the second event does not overwrite it.  Leaving
no code to actually start the whole processing.  There are probably
other cli iterations that will cause this fun as well, but I'm
not going to spend the time sussing them out at the moment.

Fixes: #12314
Signed-off-by: Donald Sharp <sharpd@nvidia.com>