Properly release key in spa_keystore_dsl_key_hold_dd()

Since dsl_crypto_key_open() references the key, 0d23f5e should
have called dsl_crypto_key_rele() to drop it first instead of
calling dsl_crypto_key_free() directly.  The final result should
actually be the same, but without triggering dck_holds assertion.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.

module/zfs/dsl_crypt.c

@@ -717,7 +717,7 @@ spa_keystore_dsl_key_hold_dd(spa_t *spa, dsl_dir_t *dd, const void *tag,
 		avl_insert(&spa->spa_keystore.sk_dsl_keys, dck_io, where);
 		*dck_out = dck_io;
 	} else {
-		dsl_crypto_key_free(dck_io);
+		dsl_crypto_key_rele(dck_io, tag);
 		*dck_out = dck_ks;
 	}