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/dir.c b/fs/fuse/dir.c
index 2b112d9..8506522 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -21,6 +21,8 @@
 
 	if (!fc->do_readdirplus)
 		return false;
+	if (!fc->readdirplus_auto)
+		return true;
 	if (test_and_clear_bit(FUSE_I_ADVISE_RDPLUS, &fi->state))
 		return true;
 	if (filp->f_pos == 0)