dmflakey: support external log and realtime devices

dmflakey: support external log and realtime devices

Upgrade the dmflakey code to coordinate making external scratch log and
scratch realtime devices flakey along with the scratch device.  Note
that unlike SCRATCH_DEV, we save the old rt/log devices as separate
variables and replace SCRATCH_{RT,LOG}DEV so that helper functions
continue to work without modification.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>