dm snapshot: use dm-bufio

Use dm-bufio for initial loading of the exceptions.
Introduce a new function dm_bufio_forget that frees the given buffer.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 7441344..39b540a 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -238,6 +238,7 @@
 config DM_SNAPSHOT
        tristate "Snapshot target"
        depends on BLK_DEV_DM
+       select DM_BUFIO
        ---help---
          Allow volume managers to take writable snapshots of a device.