block: loop: convert to blk-mq

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

Hi,

The following 6 patches convert current loop driver into blk-mq:

	- loop's scalability gets improved much
	- loop driver gets quite simplified, and the conversion can
	be throught as cleanup

The previous kernel AIO patches will be submitted in another patchset
for improving loop's performance.

V2:
	- replace work queue with kthread worker because wq may cause
	regression on some fast devices(such as loop over null_blk, tmpfs
	file, ...), in these cases, loop thread becomes CPU bound actually,
	so work queue will introduce lots of unnecessary context switch, and
	decrease throughput a lot one this cases
	- introduce prepare_flush_rq_fn callback and its pair in blk-mq ops
	for setting up flush rq's pdu correctly
	- move kernel aio patches into another patchset as suggested by
	Christoph
	- no change in the other 4 patches

V1:
	- improve failure path in aio_kernel_submit()

 block/blk-flush.c      |    4 +
 block/blk-mq.c         |   16 +-
 block/blk-mq.h         |    1 -
 drivers/block/loop.c   |  411 +++++++++++++++++++++++++-----------------------
 drivers/block/loop.h   |   20 ++-
 include/linux/blk-mq.h |   14 ++
 6 files changed, 259 insertions(+), 207 deletions(-)



Thanks,
--
Ming Lei