dm ioctl: allow dm_copy_name_and_uuid to return only one field

Allow NULL buffer in dm_copy_name_and_uuid if you only want to return one of
the fields.

(Required by a following patch that adds these fields to sysfs.)
Signed-off-by: Milan Broz <mbroz@redhat.com>
Reviewed-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-ioctl.c

@@ -1550,7 +1550,9 @@ int dm_copy_name_and_uuid(struct mapped_device *md, char *name, char *uuid)
 		goto out;
 	}
 
+	if (name)
 		strcpy(name, hc->name);
+	if (uuid)
 		strcpy(uuid, hc->uuid ? : "");
 
 out: