dm thin metadata: call precommit before saving the roots

commit 91bcdb92 upstream.

These calls were the wrong way round in __write_initial_superblock.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>