dm era: save spacemap metadata root after the pre-commit

When committing era metadata to disk, it doesn't always save the latest
spacemap metadata root in superblock. Due to this, metadata is getting
corrupted sometimes when reopening the device. The correct order of update
should be, pre-commit (shadows spacemap root), save the spacemap root
(newly shadowed block) to in-core superblock and then the final commit.

Cc: stable@vger.kernel.org
Signed-off-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>