dm: support turning off block-core's io stats accounting

Commit bc58ba94 ("block: add sysfs file for controlling io stats
accounting") allowed users to turn off disk stat accounting completely
by checking if queue flag QUEUE_FLAG_IO_STAT is set. In dm, this flag
is neither set nor checked: so block-core's io stats are continuously
counted and cannot be turned off.

Add support for turning off block-core's io stats accounting for dm.
Set QUEUE_FLAG_IO_STAT for dm's request_queue. If QUEUE_FLAG_IO_STAT
is set when an io starts, record the need for block core's io stats by
setting the DM_IO_BLK_STAT dm_io flag to avoid io stats being disabled
in the middle of the io.

DM statistics (dm-stats) is independent of block-core's io stats and
remains unchanged.

Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>