dm dust: use dust block size for badblocklist index

Change the "frontend" dust_remove_block, dust_add_block, and
dust_query_block functions to store the "dust block number", instead
of the sector number corresponding to the "dust block number".

For the "backend" functions dust_map_read and dust_map_write,
right-shift by sect_per_block_shift.  This fixes the inability to
emulate failure beyond the first sector of each "dust block" (for
devices with a "dust block size" larger than 512 bytes).

Fixes: e4f3fabd ("dm: add dust target")
Cc: stable@vger.kernel.org
Signed-off-by: Bryan Gurney <bgurney@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>