Raw DRR_OBJECT records must write raw data

b1d21733 made it possible for empty metadnode blocks to be
compressed to a hole, fixing a bug that would cause invalid
metadnode MACs when a send stream attempted to free objects
and allowing the blocks to be reclaimed when they were no
longer needed. However, this patch also introduced a race
condition; if a txg sync occurred after a DRR_OBJECT_RANGE
record was received but before any objects were added, the
metadnode block would be compressed to a hole and lose all
of its encryption parameters. This would cause subsequent
DRR_OBJECT records to fail when they attempted to write
their data into an unencrypted block. This patch defers the
DRR_OBJECT_RANGE handling to receive_object() so that the
encryption parameters are set with each object that is
written into that block.

Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7215 
Closes #7236

module/zfs/dbuf.c

@@ -3465,8 +3465,7 @@ dbuf_check_crypt(dbuf_dirty_record_t *dr)
 		 * Writing raw encrypted data requires the db's arc buffer
 		 * to be converted to raw by the caller.
 		 */
-		ASSERT(arc_is_encrypted(db->db_buf) ||
-		    db->db.db_object == DMU_META_DNODE_OBJECT);
+		ASSERT(arc_is_encrypted(db->db_buf));
 	}
 }