[BLK] Add support for barrier writes to blkfront/blkback.

[BLK] Add support for barrier writes to blkfront/blkback.

Protocol changes:
 * There is a new operation (BLKIF_OP_WRITE_BARRIER)
   to pass on barrier requests.
 * There is a new state (BLKIF_RSP_EOPNOTSUPP) to indicate
   unsupported operations (barrier writes may fail depending
   on the underlying block device).
 * A new xenstore node named "feature-barrier" indicates the
   backend is able to handle barrier writes.  The value can
   be 1 (all is fine) or 0 (underlying block device doesn't
   support barriers).

blkback changes:  Add "feature-barrier" node to indicate barrier
support, pass incoming barrier requests to the block layer using
submit_bio(WRITE_BARRIER, bio).  Some error handling fixes to
properly pass through barrier write failures, so the frontend
can turn off barriers then.

blkfront changes:  Check if the backend sets "feature-barrier", if
present switch to QUEUE_ORDERED_DRAIN mode.  Send off barrier
requests to the backend using the new BLKIF_OP_WRITE_BARRIER
operation.  Also some error handling for the EOPNOTSUPP case.

Background:  Barriers are needed to make journaling filesystems work
reliable.  For some requests they need order constrains to make the
transactions work correctly.  Barrier requests are used to pass that
ordering information to the block layer and/or to the device, so the
constrains are obeyed when reordering requests for better write
performance.

Signed-off-by: Gerd Hoffmann <kraxel@suse.de>