mtd: ioengine

This patch adds an MTD ioengine. Trims are interpreted as MTD erases.
A skip_bad option calls out to the kernel to ask if the block being
operated on is bad first; if it is bad, the operation is skipped
and -EIO is returned.

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