dm vdo: clean up scnprintf usage

Ignore scnprintf return status since it is not necessary. Change
write_* functions type from int to void since we no longer return
any result. Also, clean up any code that checks or uses any scnprintf
return results.
Check uds_allocate return code which was previous ignored, return
and log error when uds_allocate failed.
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Chung Chung <cchung@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>