zebra: Refactor 'struct rib' to be 'struct route_entry'

zebra: Refactor 'struct rib' to be 'struct route_entry'

The 'struct rib' data structure is missnamed.  It really
is a 'struct route_entry' as part of the 'struct route_node'.
We have 1 'struct route_entry' per route src.  As such
1 route node can have multiple route entries if multiple
protocols attempt to install the same route.

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