dm thin metadata: remove needless work from __commit_transaction

Commit 5a32083d ("dm: take care to copy the space map roots before
locking the superblock") properly removed the calls to dm_sm_root_size()
from __write_initial_superblock().  But the dm_sm_root_size() calls were
left dangling in __commit_transaction().

Fixes: 5a32083d ("dm: take care to copy the space map roots before locking the superblock")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>