block: move the add_random flag to queue_limits

block: move the add_random flag to queue_limits

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

Note that this also removes code from dm to clear the flag based on
the underlying devices, which can't be reached as dm devices will
always start out without the flag set.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>