Revert "Disable FOP_DONTCACHE for now due to bugs"

This reverts commit 478ad02.

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>

Author: axboe

include/linux/fs.h

@@ -2207,7 +2207,7 @@ struct file_operations {
 /* Supports asynchronous lock callbacks */
 #define FOP_ASYNC_LOCK		((__force fop_flags_t)(1 << 6))
 /* File system supports uncached read/write buffered IO */
-#define FOP_DONTCACHE		0 /* ((__force fop_flags_t)(1 << 7)) */
+#define FOP_DONTCACHE		((__force fop_flags_t)(1 << 7))
 
 /* Wrap a directory iterator that needs exclusive inode access */
 int wrap_directory_iterator(struct file *, struct dir_context *,