scsi: use sector_div instead of do_div

do_div is the wrong way to divide a sector_t, as it is less efficient
when sector_t is 32-bit wide. With the upcoming do_div optimizations,
the kernel starts warning about this:

drivers/scsi/scsi_debug.c: In function 'dif_store':
include/asm-generic/div64.h:207:28: warning: comparison of distinct pointer types lacks a cast

This changes the code to use sector_div instead, which always produces
optimal code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>