dm thin: do not send discards to shared blocks

dm thin: do not send discards to shared blocks

When process_discard receives a partial discard that doesn't cover a
full block, it sends this discard down to that block. Unfortunately, the
block can be shared and the discard would corrupt the other snapshots
sharing this block.

This patch detects block sharing and ends the discard with success when
sending it to the shared block.

The above change means that if the device supports discard it can't be
guaranteed that a discard request zeroes data. Therefore, we set
ti->discard_zeroes_data_unsupported.

Thin target discard support with this bug arrived in commit
b9d78a392ce92d7f5e280d52669e70b2ed9cce92 (dm thin: support discards).

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>