zebra: fix show route vrf all memory consumption

zebra: fix show route vrf all memory consumption

0e2fc3d67f ("vtysh, zebra: Fix malformed json output for multiple vrfs
in command 'show ip route vrf all json'") has been reverted in the
previous commit. Although the fix was correct, it was consuming too muca
memory when displaying large route tables.

A root JSON object was storing all the JSON objects containing the route
tables, each containing their respective prefixes in JSON objects. This
resulted in excessive memory allocation for JSON objects, potentially
leading to an out-of-memory error on the machine.

To Fix the memory consumption issue for the "show ip[v6] route vrf all
json" command, display the tables 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>