dm thin metadata: use struct dm_pool_metadata members in __open_or_format_metadata

Clean up __open_or_format_metadata in dm-thin-metadata by using struct
dm_pool_metadata members to replace local variables.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>