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

include/sys/dsl_crypt.h

@@ -206,6 +206,7 @@ void dsl_dataset_promote_crypt_sync(dsl_dir_t *target, dsl_dir_t *origin,
     dmu_tx_t *tx);
 int dmu_objset_create_crypt_check(dsl_dir_t *parentdd,
     dsl_crypto_params_t *dcp, boolean_t *will_encrypt);
+boolean_t dmu_objset_crypto_key_equal(objset_t *osa, objset_t *osb);
 void dsl_dataset_create_crypt_sync(uint64_t dsobj, dsl_dir_t *dd,
     struct dsl_dataset *origin, dsl_crypto_params_t *dcp, dmu_tx_t *tx);
 uint64_t dsl_crypto_key_create_sync(uint64_t crypt, dsl_wrapping_key_t *wkey,