A zone ID is a 32 bits unsigned int which can overflow when doing the
bit shifts in dmz_start_sect().  With a 256 MB zone size drive, the
overflow happens for a zone ID >= 8192.

Fix this by casting the zone ID to a sector_t before doing the bit
shift.  While at it, similarly fix dmz_start_block().
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>