dm exception store: separate type from instance

Introduce struct dm_exception_store_type.

Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --cc drivers/md/dm-exception-store.h
Simple merge

diff --cc drivers/md/dm-snap-persistent.c
Simple merge

diff --cc drivers/md/dm-snap-transient.c
Simple merge

diff --cc drivers/md/dm-snap.c
Simple merge