dm thin: prepare to separate bio_prison code

The bio prison code will be useful to share with future DM targets.

Prepare to move this code into a separate module, adding a dm prefix
to structures and functions that will be exported.
Signed-off-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>