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

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

Orabug: 25547820

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>
(cherry picked from commit 117aceb030307dcd431fdcff87ce988d3016c34a)
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>