lbmap: Use length of backend map index instead of uniqueBackends map when growing
lbmap: Use length of backend map index instead of uniqueBackends map when growing

The existing code likely worked correctly but it's not logical to use the
unique backends. Fortunately, when growing beyond holes, the number of unique
backends should always be identical to the number of backend indices.

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

cilium-team/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium