Linux 6.11: enable queue flush through queue limits

In 6.11 struct queue_limits gains a 'features' field, where, among other
things, flush and write-cache are enabled. Detect it and use it.

Along the way, the blk_queue_set_write_cache() compat wrapper gets a
little cleanup. Since both flags are alway set together, its now a
single bool. Also the very very ancient version that sets q->flush_flags
directly couldn't actually turn it off, so I've fixed that. Not that we
use it, but still.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16400