zebra: Use built in list handler for route entries now

zebra: Use built in list handler for route entries now

The route entry code was using a custom linked list to handle
route entries.  Remove and replace with the new lib link list
code.  This reduces the size of the route entry by a further
8 bytes.

Observant people will notice that the current linked list
implementation is singly linked, while the Route Entry
is doubly linked.  I am not terribly concerned about this
change as that 1) we do not see a large number of route
entries per prefix( say 2 maybe 3 items ) and route entries
do not come and go that often.

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