dm cache policy smq: allocate cache blocks in order

Previously, cache blocks were being allocated in reverse order.  Fix
this by pulling the block off the head of the free list.

Shouldn't have any impact on performance or latency but it is more
correct to have the cache blocks allocated/mapped in ascending order.
This fix will slightly increase the chances of two adjacent oblocks
being in adjacent cblocks.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>