dm snapshot: support barriers in snapshot merge target

Sets num_flush_requests=2 to support flushing both the origin and cow
devices used by the snapshot-merge target.

Also, snapshot_ctr() now gets the origin device using FMODE_WRITE if the
target is snapshot-merge (which writes to the origin device).

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>