Make DISCARD_BARRIER and DISCARD_NOBARRIER writes instead of reads

The commands are conceptually writes, and in the case of IDE and SCSI
commands actually are writes.  They were only reads because we thought
that would interact better with the elevators.  Now the elevators know
about discard requests, that advantage no longer exists.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>