dm snapshot: do not split read bios sent to snapshot-origin target

Change the snapshot-origin target so that only write bios are split on
chunk boundary.  Read bios are passed unchanged to the underlying
device, so they don't have to be split.

Later, we could change the target so that it accepts a larger write bio
if it spans an area that is completely covered by snapshot exceptions.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>