IPv6 fails with busybox: ip: either "dev" is duplicate, or "permanent" is garbage

[Benjamin Drung]

This failure was detected while working on the amd64-net autopkgtest for initramfs-tools. Create a dummy root file system and launch qemu with user network and specify ip6=dhcp on the kernel command line:

```
qemu-system-x86_64 -m 1G -drive "file=${ROOTDISK},if=${ROOTDISK_QEMU_IF},media=disk,format=raw" \
 ${USRDISK:+-drive "file=${USRDISK},if=${USRDISK_QEMU_IF},media=disk,format=raw"} \
 -nographic -no-reboot -kernel "/boot/vmlinuz-${KVER}" -initrd "${INITRAMFS}" \
 -device "virtio-net-pci,netdev=net0,bus=pci.0,addr=3,mac=52:54:00:65:43:21" \
 -netdev "user,id=net0,net=10.0.3.0/24,ipv6-net=fec7::/48,hostname=pizza,dnssearch=test,domainname=example.com,bootfile=/path/to/bootfile2" \
 -device "virtio-net-pci,netdev=net1,bus=pci.0,addr=5,mac=52:54:00:12:34:56" \
 -netdev "user,id=net1,hostname=goulash,dnssearch=example,dnssearch=example.net,domainname=test,bootfile=/path/to/bootfile" \
 -append "root=/dev/${ROOTDISK_LINUX_NAME} ro console=ttyS0,115200 ip6=dhcp"
```

/sbin/dhclient-script fails. Running the script with "set -x" shows the culprit in PREINIT6:

```
+ ip -6 addr flush dev ens5 scope global permanent
ip: either "dev" is duplicate, or "permanent" is garbage
```

The busybox ip command does not support the FLAG "permanent".

Distro: Ubuntu 23.10
Packages:
 isc-dhcp-client 4.4.3-P1-2ubuntu2
 busybox-initramfs 1:1.35.0-4ubuntu1