directmount: take fsname from opts, not cfg

This makes the directmount code path pick up this workaround,
which fixes mounting jacobsa/fuse based file systems (which
do not set the FSName explicitly, like the samples) in Docker:

https://github.com/jacobsa/fuse/blob/dd426a02d67d77d065cd0ef38126ac51092a52e0/mount_config.go#L208-L220

fixes #166

mount_linux.go

@@ -84,6 +84,7 @@ func directmount(dir string, cfg *MountConfig) (*os.File, error) {
 		mountflag = fn(mountflag)
 		delete(opts, k)
 	}
+	fsname := opts["fsname"]
 	delete(opts, "fsname") // handled via fstype mount(2) parameter
 	fstype := "fuse"
 	if subtype, ok := opts["subtype"]; ok {
@@ -96,11 +97,11 @@ func directmount(dir string, cfg *MountConfig) (*os.File, error) {
 		cfg.DebugLogger.Println("Starting the unix mounting")
 	}
 	if err := unix.Mount(
-		cfg.FSName, // source
-		dir,        // target
-		fstype,     // fstype
-		mountflag,  // mountflag
-		data,       // data
+		fsname,    // source
+		dir,       // target
+		fstype,    // fstype
+		mountflag, // mountflag
+		data,      // data
 	); err != nil {
 		if err == syscall.EPERM {
 			return nil, errFallback