Revert "Disable FOP_DONTCACHE for now due to bugs"

This reverts commit 478ad02d.

Both the read and write side dirty && writeback races should be resolved
now, revert the commit that disabled FOP_DONTCACHE for filesystems.

Link: https://lore.kernel.org/linux-fsdevel/20250525083209.GS2023217@ZenIV/


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/20250527133255.452431-4-axboe@kernel.dk


Signed-off-by: Christian Brauner <brauner@kernel.org>