dm vdo: fix sparse warnings about missing statics

Addresses various sparse warnings like:
warning: symbol 'SYMBOL' was not declared. Should it be static?

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>