lb: Fix src_ip_sticky evaluation bug in per-port-vip case.

lb: Fix src_ip_sticky evaluation bug in per-port-vip case.

Before this fix, the src_ip_sticky flag was passed as an argument to
the lb_node_get_hash function, which computes a hash value for a packet.
However, in per-port-vip case, the value of src_ip_sticky flag may be
different for each port number. As a result, the value is the same for
all port numbers, even though it is a per-port-vip case.

This commit fixes the src_ip_sticky evaluation by delaying it until the
packet is received, so that the correct value is obtained. Also, the
unit test case has been enhanced for this bug fix.

The steps to reproduce this bug are described below:
https://lists.fd.io/g/vpp-dev/message/23248

Type: fix
Fixes: 613e6dc0bf92 ("lb: add source ip based sticky load balancing")
Change-Id: I483492b214a1768e7a21fd86edd5151b3c46528b
Signed-off-by: Nobuhiro MIKI <nmiki@yahoo-corp.jp>