Allow block cloning across encrypted datasets

When two datasets share the same master encryption key, it is safe
to clone encrypted blocks. Currently only snapshots and clones
of a dataset share with it the same encryption key.

Added a test for:
- Clone from encrypted sibling to encrypted sibling with
  non encrypted parent
- Clone from encrypted parent to inherited encrypted child
- Clone from child to sibling with encrypted parent
- Clone from snapshot to the original datasets
- Clone from foreign snapshot to a foreign dataset
- Cloning from non-encrypted to encrypted datasets
- Cloning from encrypted to non-encrypted datasets

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Original-patch-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Signed-off-by: Kay Pedersen <mail@mkwg.de>
Closes #15544

man/man7/zpool-features.7

@@ -364,9 +364,12 @@ When this feature is enabled ZFS will use block cloning for operations like
 Block cloning allows to create multiple references to a single block.
 It is much faster than copying the data (as the actual data is neither read nor
 written) and takes no additional space.
-Blocks can be cloned across datasets under some conditions (like disabled
-encryption and equal
-.Nm recordsize ) .
+Blocks can be cloned across datasets under some conditions (like equal
+.Nm recordsize ,
+the same master encryption key, etc.).
+ZFS tries its best to clone across datasets including encrypted ones.
+This is limited for various (nontrivial) reasons depending on the OS
+and/or ZFS internals.
 .Pp
 This feature becomes
 .Sy active