dm log: add flush callback fn

Introduce a callback pointer from the log to dm-raid1 layer.

Before some region is set as "in-sync", we need to flush hardware cache on
all the disks. But the log module doesn't have access to the mirror_set
structure. So it will use this callback.

So far the callback is unused, it will be used in further patches.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>