Add Linux namespace delegation support

This allows ZFS datasets to be delegated to a user/mount namespace
Within that namespace, only the delegated datasets are visible
Works very similarly to Zones/Jailes on other ZFS OSes

As a user:
```
 $ unshare -Um
 $ zfs list
no datasets available
 $ echo $$
1234
```

As root:
```
 # zfs list
NAME                            ZONED  MOUNTPOINT
containers                      off    /containers
containers/host                 off    /containers/host
containers/host/child           off    /containers/host/child
containers/host/child/gchild    off    /containers/host/child/gchild
containers/unpriv               on     /unpriv
containers/unpriv/child         on     /unpriv/child
containers/unpriv/child/gchild  on     /unpriv/child/gchild

 # zfs zone /proc/1234/ns/user containers/unpriv
```

Back to the user namespace:
```
 $ zfs list
NAME                             USED  AVAIL     REFER  MOUNTPOINT
containers                       129M  47.8G       24K  /containers
containers/unpriv                128M  47.8G       24K  /unpriv
containers/unpriv/child          128M  47.8G      128M  /unpriv/child
```

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Will Andrews <[email protected]>
Signed-off-by: Allan Jude <[email protected]>
Signed-off-by: Mateusz Piotrowski <[email protected]>
Co-authored-by: Allan Jude <[email protected]>
Co-authored-by: Mateusz Piotrowski <[email protected]>
Sponsored-by: Buddy <https://buddy.works>
Closes #12263

1 files changed, 2 insertions, 0 deletions

diff --git a/man/Makefile.am b/man/Makefile.am
index 8fa21d2fd..12f818372 100644
--- a/man/Makefile.am
+++ b/man/Makefile.am
@@ -59,9 +59,11 @@ dist_man_MANS = \
 	%D%/man8/zfs-unjail.8 \
 	%D%/man8/zfs-unload-key.8 \
 	%D%/man8/zfs-unmount.8 \
+	%D%/man8/zfs-unzone.8 \
 	%D%/man8/zfs-upgrade.8 \
 	%D%/man8/zfs-userspace.8 \
 	%D%/man8/zfs-wait.8 \
+	%D%/man8/zfs-zone.8 \
 	%D%/man8/zfs_ids_to_path.8 \
 	%D%/man8/zgenhostid.8 \
 	%D%/man8/zinject.8 \