verify/trim: stop issuing trims if we run out

verify/trim: stop issuing trims if we run out

If we have drained the list of trim operations but its original contents
were fewer than a full batch we should zero out the running batch count
to make sure that we issue another full set of trim_backlog write
operations before considering trims again. Otherwise we will immediately
trim after each subsequent write operation until we have met the batch
size requirement.

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