dm thin metadata: lift __begin_transaction out of __write_initial_superblock

Lift the call to __begin_transaction out of __write_initial_superblock in
dm-thin-metadata.  Called higher up the call chain now.

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>