Add new writetrim rw= mode for trims preceding writes

Add new writetrim rw= mode for trims preceding writes

In this new mode, sequential trims and writes are interspersed by first
doing a trim at a particular offset, then doing writes starting from
that offset until the start of the next trim block, then another trim,
etc. This workload is designed to match the requirements of NAND flash,
if trims are implemented as erases.

Signed-off-by: Dan Ehrenberg <dehrenberg@chromium.org>
Signed-off-by: Jens Axboe <axboe@fb.com>