randtrimwrite: fix corner case with variable block sizes

randtrimwrite: fix corner case with variable block sizes

When we have variable block sizes it's possible to finish a trim + write
pair and then have the next (smaller length) trim operation have a
different start offset but the same end offset as the previous pair of
trim and write operations. This would fool fio into believing that it
had already completed a trim + write pair when it actually completed
only the trim.

Resolve this problem by comparing start offsets instead of end offsets.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>