Linux configurer crashes when resolving IP has 255.255.255.255 netmask

[andrelaszlo]

iproute2 outputs just the ip without subnet when the netmask is 255.255.255.255

This causes an issue in the Linux configurer (running with --debug):

INFO[0001] ==> Running phase: Gather host facts 
DEBU[0001] [local] localhost: executing `uname -m`      
DEBU[0001] [local] localhost: x86_64                    
DEBU[0001] [local] localhost: executing `hostname 2> /dev/null` 
DEBU[0001] [local] localhost: myhostname                  
INFO[0001] [local] localhost: using myhostname as hostname 
DEBU[0001] [local] localhost: executing `PATH=/usr/local/sbin:/usr/sbin:/sbin:$PATH; (ip route list scope global | grep -E "\b(172|10|192\.168)\.") || (ip route list | grep -m1 default)` 
DEBU[0001] [local] localhost: default via 12.34.56.1 dev enp2s0 proto static 
INFO[0001] [local] localhost: discovered enp2s0 as private interface 
DEBU[0001] [local] localhost: executing `PATH=/usr/local/sbin:/usr/sbin:/sbin:$PATH ip -o addr show dev enp2s0 scope global` 
DEBU[0001] [local] localhost: 2: enp2s0    inet 12.34.56.78 peer 12.34.56.1/32 scope global enp2s0\       valid_lft forever preferred_lft forever 
DEBU[0001] [local] localhost: 2: enp2s0    inet6 [redacted]/64 scope global \       valid_lft forever preferred_lft forever 
panic: runtime error: slice bounds out of range [:-1]

goroutine 31 [running]:
github.com/k0sproject/k0sctl/configurer.Linux.PrivateAddress({{0xc0001f3c00, 0xbfc340}}, {0xd31b20, 0xc0000c6000}, {0xc000628340, 0x6}, {0xc20f1f, 0x9})
	/path/to/project/k0sctl/configurer/linux.go:178 +0x308
github.com/k0sproject/k0sctl/phase.(*GatherFacts).investigateHost(0xc0000b0b40, 0xc0000c6000)
	/path/to/project/k0sctl/phase/gather_facts.go:65 +0x4c3
github.com/k0sproject/k0sctl/pkg/apis/k0sctl.k0sproject.io/v1beta1/cluster.Hosts.ParallelEach.func1(0xc0000c6000)
	/path/to/project/k0sctl/pkg/apis/k0sctl.k0sproject.io/v1beta1/cluster/hosts.go:115 +0xcb
created by github.com/k0sproject/k0sctl/pkg/apis/k0sctl.k0sproject.io/v1beta1/cluster.Hosts.ParallelEach
	/path/to/project/k0sctl/pkg/apis/k0sctl.k0sproject.io/v1beta1/cluster/hosts.go:114 +0x25a

(Some values changed for privacy reasons.)

If I am reading this correctly, the issue is that the CIDR notation is omitted and the IP field simply doesn't have a slash.

Suggested fix: check that strings.Index(items[3], "/") is nonnegative, otherwise use the field as-is.
Better fix might be to parse ips json output instead. Seems like it's been around since 2017 or so.

Affected version(s): verified in 0.12.0 and 0.12.1