fuse: add flag to turn on async direct IO

Without async DIO write requests to a single file were always serialized.
With async DIO that's no longer the case.

So don't turn on async DIO by default for fear of breaking backward
compatibility.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/fuse_i.h

@@ -541,6 +541,9 @@ struct fuse_conn {
 	/** Does the filesystem want adaptive readdirplus? */
 	unsigned readdirplus_auto:1;
 
+	/** Does the filesystem support asynchronous direct-IO submission? */
+	unsigned async_dio:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;