block: move the io_stat flag setting to queue_limits

block: move the io_stat flag setting to queue_limits

Move the io_stat flag into the queue_limits feature field so that it can
be set atomically with the queue frozen.

Simplify md and dm to set the flag unconditionally instead of avoiding
setting a simple flag for cases where it already is set by other means,
which is a bit pointless.

Signed-off-by: Christoph Hellwig <hch@lst.de>