block: Convert RQF_PREEMPT into REQ_PREEMPT

This patch does not change any functionality but makes the
REQ_PREEMPT flag available to blk_get_request(). A later patch
will add code to blk_get_request() that checks the REQ_PREEMPT
flag. Note: the IDE sense_rq request is allocated statically so
there is no blk_get_request() call that corresponds to this
request.

Change-Id: I380e869515f106e882c03b5305dc8e675eefd915
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Patch-mainline: linux-block@vger.kernel.org @ 03/10/2017, 22:04
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>