Catch the case where size= is less than the minimum block size

Catch the case where size= is less than the minimum block size

Fio will currently just attempt to start and immediately exit,
with no clue as to what went wrong. The behaviour is correct
in the sense that there's nothing to output, but it can be a
little confusing.

Signed-off-by: Jens Axboe <axboe@kernel.dk>