block: fix DISCARD_BARRIER requests

Filesystems assume that DISCARD_BARRIER are full barriers, so that they
don't have to track in-progress discard operation when submitting new I/O.
But currently we only treat them as elevator barriers, which don't
actually do the nessecary queue drains.

Also remove the unlikely around both the DISCARD and BARRIER requests -
the happen far too often for a static mispredict.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>