dm thin: fix passdown_double_checking_shared_status()

Commit 00a0ea33 ("dm thin: do not queue freed thin mapping for next
stage processing") changed process_prepared_discard_passdown_pt1() to
increment all the blocks being discarded until after the passdown had
completed to avoid them being prematurely reused.

IO issued to a thin device that breaks sharing with a snapshot, followed
by a discard issued to snapshot(s) that previously shared the block(s),
results in passdown_double_checking_shared_status() being called to
iterate through the blocks double checking their reference count is zero
and issuing the passdown if so.  So a side effect of commit 00a0ea33
is passdown_double_checking_shared_status() was broken.

Fix this by checking if the block reference count is greater than 1.
Also, rename dm_pool_block_is_used() to dm_pool_block_is_shared().

Fixes: 00a0ea33 ("dm thin: do not queue freed thin mapping for next stage processing")
Cc: stable@vger.kernel.org # 4.9+
Reported-by:  <ryan.p.norwood@gmail.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>