zebra: fix show route memory consumption

zebra: fix show route memory consumption

When displaying a route table in JSON, a table JSON object is storing
all the prefix JSON objects containing the prefix information. This
results in excessive memory allocation for JSON objects, potentially
leading to an out-of-memory error on the machine with large routing
tables.

To Fix the memory consumption issue for the "show ip[v6] route [vrf XX]
json" command, display the prefixes one by one and free the memory of
each JSON object after it has been displayed.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>