Re-introduce RWF_DONTCACHE

Re-introduce RWF_DONTCACHE

This used to be called RWF_UNCACHED, and it never made it upstream. But
as of the 6.14 kernel, RWF_DONTCACHE exists, and provides the same
guarantees that the older RWF_UNCACHED did - it's applied to buffered
IO, and any page cache instantiated for this read or write will be
dropped on IO completion. Any data already in cache will remain in cache
and will not cause IO to be issued.

This adds support for the io_uring and sync IO engines.

Signed-off-by: Jens Axboe <axboe@kernel.dk>