zfs unmount legacy filesystem fails
                     Hello,
                key problem: I created a legacy mount point, but am not able to umount it again.
first, I wanted to create a mirrored pool, with mount point /zones:
# zpool create zones_pool mirror c1t6000D7760000BA34537FCCCF86DAB2EDd0 c1t6000D77600004FE05380CCCF7F7D064Ed0
# zfs create zones_pool/zones
# zfs set mountpoint=/zones zones_pool/zones
# zfs list
NAME               USED  AVAIL  REFER  MOUNTPOINT
rpool             17.9G  72.7G  42.5K  /rpool
rpool/ROOT        9.14G  72.7G    31K  legacy
rpool/ROOT/zfsBE  9.14G  72.7G  9.14G  /
rpool/dump        4.50G  72.7G  4.50G  -
rpool/swap        4.25G  72.9G  4.00G  -
zones_pool         142K  97.9G    32K  /zones_pool
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