fuse: allow control of adaptive readdirplus use

For some filesystems (e.g. GlusterFS), the cost of performing a
normal readdir and readdirplus are identical.  Since adaptively
using readdirplus has no benefit for those systems, give
users/filesystems the option to control adaptive readdirplus use.

v2 of this patch incorporates Miklos's suggestion to simplify the code,
as well as improving consistency of macro names and documentation.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index fc55dd3..6aeba86 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -514,9 +514,12 @@
 	/** Use enhanced/automatic page cache invalidation. */
 	unsigned auto_inval_data:1;
 
-	/** Does the filesystem support readdir-plus? */
+	/** Does the filesystem support readdirplus? */
 	unsigned do_readdirplus:1;
 
+	/** Does the filesystem want adaptive readdirplus? */
+	unsigned readdirplus_auto:1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;