fix #4440: cherry-pick patch for mountpoint=legacy on /

fix #4440: cherry-pick patch for mountpoint=legacy on /

As reported having set 'mountpoint=legacy' on the root dataset (for a
default install this would be 'rpool/ROOT/pve-1') for mounting through
e.g. /etc/fstab breaks booting in initramfs

The cherry-picked patch is already applied to zfs-2.1.8-staging
upstream so will be included in the next zfs point release.

reproduced the issue on a VM with ZFS on root by:
* booting into a PVE 7.3 iso (to have zfs and an inactive
  rpool/ROOT/pve-1)
* `zpool import -ocachefile=none -N -f rpool
* `zfs set mountpoint=legacy rpool/ROOT/pve-1`
* `mount -t zfs rpool/ROOT/pve-1 /mnt
* `echo "rpool/ROOT/pve-1 / zfs defaults 0 0" >> /mnt/etc/fstab`
* `zpool export rpool`
* reboot - the boot breaks with zfs-initramfs 2.1.7-pve1 but works
  with this patch applied

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>