block: deprecate barrier and replace blk_queue_ordered() with blk_queue_flush()

Barrier is deemed too heavy and will soon be replaced by FLUSH/FUA
requests.  Deprecate barrier.  All REQ_HARDBARRIERs are failed with
-EOPNOTSUPP and blk_queue_ordered() is replaced with simpler
blk_queue_flush().

blk_queue_flush() takes combinations of REQ_FLUSH and FUA.  If a
device has write cache and can flush it, it should set REQ_FLUSH.  If
the device can handle FUA writes, it should also set REQ_FUA.

All blk_queue_ordered() users are converted.

* ORDERED_DRAIN is mapped to 0 which is the default value.
* ORDERED_DRAIN_FLUSH is mapped to REQ_FLUSH.
* ORDERED_DRAIN_FLUSH_FUA is mapped to REQ_FLUSH | REQ_FUA.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Boaz Harrosh <bharrosh@panasas.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Alasdair G Kergon <agk@redhat.com>
Cc: Pierre Ossman <drzeus@drzeus.cx>
Cc: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

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