btrfs: add support for uncached writes

btrfs: add support for uncached writes

The read side is already covered as btrfs uses the generic filemap
helpers. For writes, just pass in FGP_DONTCACHE if uncached IO is being
done, then the folios created should be marked appropriately.

For IO completion, ensure that writing back folios that are uncached
gets punted to one of the btrfs workers, as task context is needed for
that. Add an 'uncached_io' member to struct btrfs_bio to manage that.

With that, add FOP_DONTCACHE to the btrfs file_operations fop_flags
structure, enabling use of RWF_DONTCACHE.

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