cfq-iosched: fix sequential write regression

We have a 10-15% performance regression for sequential writes on TCQ/NCQ
enabled drives in 2.6.21-rcX after the CFQ update went in.  It has been
reported by Valerie Clement <valerie.clement@bull.net> and the Intel
testing folks.  The regression is because of CFQ's now more aggressive
queue control, limiting the depth available to the device.

This patches fixes that regression by allowing a greater depth when only
one queue is busy.  It has been tested to not impact sync-vs-async
workloads too much - we still do a lot better than 2.6.20.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>