[SCSI] make scsi_queue_insert() use blk_requeue_request()

scsi_queue_insert() used to use blk_insert_request() for requeueing
requests.  This depends on the unobvious behavior of
blk_insert_request() setting REQ_SPECIAL and REQ_SOFTBARRIER when
requeueing.  This patch makes scsi_queue_insert() use
blk_requeue_request().  As REQ_SPECIAL means special requests and
REQ_SOFTBARRIER is automatically handled by blk layer now, no flag
needs to be set.

Note that scsi_queue_insert() now calls scsi_run_queue() itself, and
the prototype of the function is added right above
scsi_queue_insert().  This is temporary, as later requeue path
consolidation patchset removes scsi_queue_insert().  By adding
temporary prototype, we can do away with unnecessarily moving
functions.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>