bgpd: evpn json cli bug fix

bgpd: evpn json cli bug fix

Bug: If there are 2 different prefixes under an rd, the
output of "show bgp l2vpn evpn json" would print only one of the
prefixes.

RCA: prefix info was added to the json_object once per rd. Hence,
prefix and rd were added just once, as the loop iterated over all
the prefixes and paths.

This is related to my earlier commit that went in via PR 4283:
https://github.com/FRRouting/frr/pull/4283

Signed-off-by: Lakshman Krishnamoorthy <lkrishnamoor@vmware.com>