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, 1 deletions

diff --git a/module/Kbuild.in b/module/Kbuild.in
index ed8dc23a9..14f236281 100644
--- a/module/Kbuild.in
+++ b/module/Kbuild.in
@@ -65,7 +65,8 @@ SPL_OBJS := \
 	spl-tsd.o \
 	spl-vmem.o \
 	spl-xdr.o \
-	spl-zlib.o
+	spl-zlib.o \
+	spl-zone.o
 
 spl-objs += $(addprefix os/linux/spl/,$(SPL_OBJS))