lb: Fix generating illegal key in per-port vip
lb: Fix generating illegal key in per-port vip

VIP prefix index becomes always 0 when adding a VIP which is already registered different port, causing LB config crash.
This change assigns the same VIP prefix index to the same VIP.

Ticket: https://jira.fd.io/browse/VPP-1834
Type: fix

Signed-off-by: Yasuhiro Nakamura <069e389def49ff73774aec680baac3ac461c196e@yahoo-corp.jp>
Change-Id: Ib63b3e58db9bd85714d68cd1292aadd0c8580da8

chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp