common/ext4: reformat external logs during mdrestore operations

common/ext4: reformat external logs during mdrestore operations

The e2image file format doesn't support the capture of external log
devices, which means that mdrestore ought to reformat the external log
to get the restored filesystem to work again.  The common/populate code
could already do this, so push it to the common ext4 helper.

While we're at it, fix the uncareful usage of SCRATCH_LOGDEV in the
populate code.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>