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 <behlendorf1@llnl.gov>
Signed-off-by: Will Andrews <will.andrews@klarasystems.com>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Mateusz Piotrowski <mateusz.piotrowski@klarasystems.com>
Co-authored-by: Allan Jude <allan@klarasystems.com>
Co-authored-by: Mateusz Piotrowski <mateusz.piotrowski@klarasystems.com>
Sponsored-by: Buddy <https://buddy.works>
Closes #12263

include/libzfs.h

@@ -150,6 +150,7 @@ typedef enum zfs_error {
 	EZFS_EXPORT_IN_PROGRESS,	/* currently exporting the pool */
 	EZFS_REBUILDING,	/* resilvering (sequential reconstrution) */
 	EZFS_VDEV_NOTSUP,	/* ops not supported for this type of vdev */
+	EZFS_NOT_USER_NAMESPACE,	/* a file is not a user namespace */
 	EZFS_UNKNOWN
 } zfs_error_t;
 
@@ -979,6 +980,15 @@ _LIBZFS_H int zpool_nextboot(libzfs_handle_t *, uint64_t, uint64_t,
 
 #endif /* __FreeBSD__ */
 
+#ifdef __linux__
+
+/*
+ * Add or delete the given filesystem to/from the given user namespace.
+ */
+_LIBZFS_H int zfs_userns(zfs_handle_t *zhp, const char *nspath, int attach);
+
+#endif
+
 #ifdef	__cplusplus
 }
 #endif

include/os/linux/spl/sys/zone.h

@@ -25,11 +25,34 @@
 #define	_SPL_ZONE_H
 
 #include <sys/byteorder.h>
+#include <sys/cred.h>
 
-#define	GLOBAL_ZONEID			0
+#include <linux/cred.h>
+#include <linux/user_namespace.h>
 
-#define	zone_dataset_visible(x, y)	(1)
-#define	crgetzoneid(x)			(GLOBAL_ZONEID)
-#define	INGLOBALZONE(z)			(1)
+/*
+ * Attach the given dataset to the given user namespace.
+ */
+extern int zone_dataset_attach(cred_t *, const char *, int);
+
+/*
+ * Detach the given dataset from the given user namespace.
+ */
+extern int zone_dataset_detach(cred_t *, const char *, int);
+
+/*
+ * Returns true if the named pool/dataset is visible in the current zone.
+ */
+extern int zone_dataset_visible(const char *dataset, int *write);
+
+int spl_zone_init(void);
+void spl_zone_fini(void);
+
+extern unsigned int crgetzoneid(const cred_t *);
+extern unsigned int global_zoneid(void);
+extern boolean_t inglobalzone(proc_t *);
+
+#define	INGLOBALZONE(x) inglobalzone(x)
+#define	GLOBAL_ZONEID	global_zoneid()
 
 #endif /* SPL_ZONE_H */

include/sys/fs/zfs.h

@@ -1450,7 +1450,9 @@ typedef enum zfs_ioc {
 	ZFS_IOC_EVENTS_SEEK,			/* 0x83 (Linux) */
 	ZFS_IOC_NEXTBOOT,			/* 0x84 (FreeBSD) */
 	ZFS_IOC_JAIL,				/* 0x85 (FreeBSD) */
+	ZFS_IOC_USERNS_ATTACH = ZFS_IOC_JAIL,	/* 0x85 (Linux) */
 	ZFS_IOC_UNJAIL,				/* 0x86 (FreeBSD) */
+	ZFS_IOC_USERNS_DETACH = ZFS_IOC_UNJAIL,	/* 0x86 (Linux) */
 	ZFS_IOC_SET_BOOTENV,			/* 0x87 */
 	ZFS_IOC_GET_BOOTENV,			/* 0x88 */
 	ZFS_IOC_LAST
@@ -1531,6 +1533,7 @@ typedef enum {
 	ZFS_ERR_REBUILD_IN_PROGRESS,
 	ZFS_ERR_BADPROP,
 	ZFS_ERR_VDEV_NOTSUP,
+	ZFS_ERR_NOT_USER_NAMESPACE,
 } zfs_errno_t;
 
 /*