dm vdo: cleanup style for comments in structs

Use /* ... */ rather than /** ... */ if for no other reason than
syntax highlighting is improved (at least for me, in emacs: comments
are now red, code is yellow. Previously comments were also yellow).

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>