[PATCH v2 0/9] block: per-distpatch_queue flush machinery

[Ming Lei <ming.lei@canonical.com>]

Hi,

As recent discussion, especially suggested by Christoph, this patchset
implements per-distpatch_queue flush machinery, so that:

	- current init_request and exit_request callbacks can
	cover flush request too, then the buggy copying way of
	initializing flush request's pdu can be fixed

	- flushing performance gets improved in case of multi hw-queue

About 70% throughput improvement is observed in sync write/randwrite
over multi dispatch-queue virtio-blk, see details in commit log
of patch 9/9.

This patchset can be pulled from below tree too:

        git://kernel.ubuntu.com/ming/linux.git v3.17-block-dev_v2

V2:
	- refactor blk_mq_init_hw_queues() and its pair, also it is a fix
	on failure path, so that conversion to per-queue flush becomes simple.
	- allocate/initialize flush queue in blk_mq_init_hw_queues()
	- add sync write tests on virtio-blk which is backed by SSD image

V1:
        - commit log typo fix
        - introduce blk_alloc_flush_queue() and its pair earlier, so
        that patch 5 and 8 become easier for review