dmerror: support external log and realtime devices

dmerror: support external log and realtime devices

Upgrade the dmerror code to coordinate making external scratch log and
scratch realtime devices error out along with the scratch device.  Note
that unlike SCRATCH_DEV, we save the old rt/log devices in a separate
variable and overwrite SCRATCH_{RT,LOG}DEV so that all the helper
functions continue to work properly.

This is very similar to what we did for dm-flakey a while back.

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