ext4: allow ext4 to use $SCRATCH_LOGDEV

ext4: allow ext4 to use $SCRATCH_LOGDEV

The writeback error handling test requires that you put the journal on a
separate device. This allows us to use dmerror to simulate data
writeback failure, without affecting the journal.

xfs already has infrastructure for this (a'la $SCRATCH_LOGDEV), so wire
up the ext4 code so that it can do the same thing when _scratch_mkfs is
called.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>