block: add plug for blkdev_issue_discard

Last post of this patch appears lost, so I resend this.

Now discard merge works, add plug for blkdev_issue_discard. This will help
discard request merge especially for raid0 case. In raid0, a big discard
request is split to small requests, and if correct plug is added, such small
requests can be merged in low layer.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>