netfilter: ipvs: Keep latest weight of destination

The hashing table in scheduler such as source hash or maglev hash
should ignore the changed weight to 0 and allow changing the weight
from/to non-0 values. So, struct ip_vs_dest needs to keep weight
with latest non-0 weight.

Signed-off-by: Inju Song <inju.song@navercorp.com>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>