fix: assign IPv6 podCIDR from kubelet spec.

[sergelogvinov]

Description

Fix segfault loop if flannel does not have IPv6-cidr in the config.

I0528 13:35:18.994988       1 vxlan.go:138] VXLAN config: VNI=1 Port=4789 GBP=false Learning=false DirectRouting=false
I0528 13:35:18.995958       1 kube.go:351] Setting NodeNetworkUnavailable
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x738dfd]

goroutine 1 [running]:
github.com/flannel-io/flannel/pkg/ip.(*IP6).ToIP(0x0)
	/go/src/github.com/flannel-io/flannel/pkg/ip/ip6net.go:82 +0x1d
github.com/flannel-io/flannel/pkg/ip.IP6Net.ToIPNet({0x0, 0xc00015a7b0})
	/go/src/github.com/flannel-io/flannel/pkg/ip/ip6net.go:167 +0x27
github.com/flannel-io/flannel/subnet/kube.(*kubeSubnetManager).AcquireLease(0xc000528380, {0x1eb1a18, 0xc00007dac0}, 0xc000108cd0)
	/go/src/github.com/flannel-io/flannel/subnet/kube/kube.go:372 +0x1a79
github.com/flannel-io/flannel/backend/vxlan.(*VXLANBackend).RegisterNetwork(0xc0004f9320, {0x1eb1a18, 0xc00007dac0}, 0x0, 0xc00056c680)
	/go/src/github.com/flannel-io/flannel/backend/vxlan/vxlan.go:181 +0x529
main.main()
	/go/src/github.com/flannel-io/flannel/main.go:324 +0xae8

Todos

• Tests
• Documentation
• Release note

Release Note

v0.18.0