dm thin: fix to consistently zero-fill reads to unprovisioned blocks

It was always intended that a read to an unprovisioned block will return
zeroes regardless of whether the pool is in read-only or read-write
mode.  thin_bio_map() was inconsistent with its handling of such reads
when the pool is in read-only mode, it now properly zero-fills the bios
it returns in response to unprovisioned block reads.

Eliminate thin_bio_map()'s special read-only mode handling of -ENODATA
and just allow the IO to be deferred to the worker which will result in
pool->process_bio() handling the IO (which already properly zero-fills
reads to unprovisioned blocks).

Reported-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

drivers/md/dm-thin.c

@@ -2358,17 +2358,6 @@ static int thin_bio_map(struct dm_target *ti, struct bio *bio)
  return DM_MAPIO_REMAPPED;
 
  case -ENODATA:
- if (get_pool_mode(tc->pool) == PM_READ_ONLY) {
- /*
- * This block isn't provisioned, and we have no way
- * of doing so.
- */
- handle_unserviceable_bio(tc->pool, bio);
- cell_defer_no_holder(tc, virt_cell);
- return DM_MAPIO_SUBMITTED;
- }
- /* fall through */
-
  case -EWOULDBLOCK:
  thin_defer_cell(tc, virt_cell);
  return DM_MAPIO_SUBMITTED;