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 --cc drivers/md/Kconfig
Simple merge

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

diff --cc drivers/md/dm-bufio.h
Simple merge

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