dm thin metadata: factor out __write_initial_superblock

Factor out __write_initial_superblock and also pull some other initial
creation code out of dm_pool_metadata_open.

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>