Handle 32-bit overflows in disk utilization stats

Handle 32-bit overflows in disk utilization stats

Linux prints [1] some of the values reported in block device's `stat`
sysfs file as 32-bit unsigned integers. fio interprets them as 64-bit
integers when reading that sysfs file and performs further arithmetics
on them in 64-bits. If the reported value overflows during fio run,
a huge bogus value is reported in the "disk utilization" block instead.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/genhd.c#n962

Signed-off-by: Alexey Neyman <aneyman@google.com>