block: drop duplicated slice extension code

The current slice is extended when an I/O request exceeds the limit.
There is no need to extend the slice every time we check a request.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-By: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
