dm vdo flush: initialize return to NULL in allocate_flush

Otherwise, error path could result in allocate_flush's subsequent
check for flush being non-NULL leading to false positive.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Susan LeGendre-McGhee <slegendr@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>