as-iosched deals with aliased requests differently from other ioscheds.

It links together aliased requests using rq->queuelist instead of
spilling alises to dispatch queue like other ioscheds do.  Requests
linked in this way cannot be merged.

Unfortunately, generic q->last_merge handling patch didn't take this
into account and q->last_merge could be set to an aliased request
resulting in Badness, corrupt list and eventually panic.

This explicitly marks aliased requests to be unmergeable.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>