From: Jens Axboe <axboe@suse.de>

AS does not correctly account requests inserted with INSERT_FRONT or
INSERT_BACK, barriers for example.  In other elevators, requeued requests also
go through the insert path, but AS has its own requeue handler which means the
code has never been tested.

Also, make inserting a barrier with INSERT_SORT imply INSERT_BACK, which is
the logical behaviour.  Previously such insertions weren't rigorously defined.