The noop io scheduler has a data corrupting bug, because q->last_merge
doesn't get cleared properly.  So do that in io scheduler core, and
remove the same code from deadline.

Also kill bio_rq_in_between(), it's not used by anyone anymore. rbtrees
are the hot thing these days.

And finally, remove a direct test for REQ_CMD in rq flags, use
blk_fs_request() instead.