populate: reformat external ext[34] journal devices when restoring a cached image

populate: reformat external ext[34] journal devices when restoring a cached image

The fs population code has the ability to save cached metadumps of
filesystems to save time when running fstests.  The cached images should
be unmounted cleanly, so we never save the contents of external journal
devices.

Unfortunately, the cache restore code fails to reset the external
journal when restoring a clean image, so we ignore cached images because
the journal doesn't match the filesystem.  This makes test runtimes
longer than they need to be.

Solve this by reformatting the external journal to match the filesystem.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>