Added encryption support for zfs recv -o / -x

One small integration that was absent from b52563 was
support for zfs recv -o / -x with regards to encryption
parameters. The main use cases of this are as follows:

* Receiving an unencrypted stream as encrypted without
  needing to create a "dummy" encrypted parent so that
  encryption can be inheritted.

* Allowing users to change their keylocation on receive,
  so long as the receiving dataset is an encryption root.

* Allowing users to explicitly exclude or override the
  encryption property from an unencrypted properties stream,
  allowing it to be received as encrypted.

* Receiving a recursive heirarchy of unencrypted datasets,
  encrypting the top-level one and forcing all children to
  inherit the encryption.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7650

module/zfs/dsl_crypt.c

@@ -1719,6 +1719,10 @@ dmu_objset_create_crypt_check(dsl_dir_t *parentdd, dsl_crypto_params_t *dcp)
 {
 	int ret;
 	uint64_t pcrypt, crypt;
+	dsl_crypto_params_t dummy_dcp = { 0 };
+
+	if (dcp == NULL)
+		dcp = &dummy_dcp;
 
 	if (dcp->cp_cmd != DCP_CMD_NONE)
 		return (SET_ERROR(EINVAL));