dm vdo block-map: rename page state name from "UDS_FREE" to "FREE"

Only used for log message, but no need for "UDS_" prefix.

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