dm thin: support a non power of 2 discard_granularity

Support a non-power-of-2 discard granularity in dm-thin, now that the block
layer supports this(via 8dd2cb7e880d2f77fba53b523c99133ad5054cfd "block:
discard granularity might not be power of 2" and
59771079c18c44e39106f0f30054025acafadb41 "blk: avoid divide-by-zero with zero
discard granularity").

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