dm vdo: check for VDO_SUCCESS return value from memory-alloc functions

VDO_SUCCESS and UDS_SUCCESS were used interchangably, update all
callers of VDO's memory-alloc functions to consistently check for
VDO_SUCCESS.

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