Set default_permissions when mounting.

This re-fixes MemFSTest.Mkdir_PermissionDenied on Linux after the recent
shakeup when changing the interface of Mount.

file_system.go

@@ -262,9 +262,10 @@ type InodeAttributes struct {
 	// The mode of the inode. This is exposed to the user in e.g. the result of
 	// fstat(2).
 	//
-	// The permissions bits of the mode are not necessarily respected by the FUSE
-	// kernel layer unless the file system is mounted with the
-	// 'default_permissions' option (cf. http://goo.gl/1LxOop).
+	// Note that in contrast to the defaults for FUSE, this package mounts file
+	// systems in a manner such that the kernel checks inode permissions in the
+	// standard posix way. This is implemented by setting the default_permissions
+	// mount option (cf. http://goo.gl/1LxOop and http://goo.gl/1pTjuk).
 	//
 	// For example, in the case of mkdir:
 	//

mounted_file_system.go

@@ -121,6 +121,12 @@ type MountConfig struct {
 
 // Convert to mount options to be passed to package bazilfuse.
 func (c *MountConfig) bazilfuseOptions() (opts []bazilfuse.MountOption) {
+	opts = []bazilfuse.MountOption{
+		// Enable permissions checking in the kernel. See the comments on
+		// InodeAttributes.Mode.
+		bazilfuse.SetOption("default_permissions", ""),
+	}
+
 	return
 }