dm vdo: fix how dm_kcopyd_client_create() failure is checked

dm_kcopyd_client_create() returns an ERR_PTR so its return must be
checked with IS_ERR().

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Chung Chung <cchung@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>