dm snapshot: flush merged data before committing metadata

If the origin device has a volatile write-back cache and the following
events occur:

1: After finishing merge operation of one set of exceptions,
   merge_callback() is invoked.
2: Update the metadata in COW device tracking the merge completion.
   This update to COW device is flushed cleanly.
3: System crashes and the origin device's cache where the recent
   merge was completed has not been flushed.

During the next cycle when we read the metadata from the COW device,
we will skip reading those metadata whose merge was completed in
step (1). This will lead to data loss/corruption.

To address this, flush the origin device post merge IO before
updating the metadata.

Cc: [email protected]
Signed-off-by: Akilesh Kailash <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>

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