Tidy the transaction manager creation functions.

They no longer lock the superblock.  Superblock locking is pulled out to
the caller.

Also export dm_bm_write_lock_zero.
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>