scsi: tcmu: use u64 for dev_size

We use unsigned long, size_t and u64 for dev_size. This has us standardize
on u64.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>