dirstream_linux: fix zero terminator search

The old code searched for the first non-null byte from the end of
the slice. This assumes that all bytes after the name are initialized
to zero, which does not hold true on Linux 4.15.

Instead, search for the first null byte from the start of the slice,
which is guaranteed by man(3) readdir.

Fixes https://github.com/hanwen/go-fuse/issues/287

nodefs/dirstream_linux.go

@@ -63,12 +63,14 @@ func (ds *loopbackDirStream) Next() (fuse.DirEntry, syscall.Errno) {
 	nameBytes := ds.todo[unsafe.Offsetof(syscall.Dirent{}.Name):de.Reclen]
 	ds.todo = ds.todo[de.Reclen:]
 
-	for l := len(nameBytes); l > 0; l-- {
-		if nameBytes[l-1] != 0 {
+	// After the loop, l contains the index of the first '\0'.
+	l := 0
+	for l = range nameBytes {
+		if nameBytes[l] == 0 {
 			break
 		}
-		nameBytes = nameBytes[:l-1]
 	}
+	nameBytes = nameBytes[:l]
 	result := fuse.DirEntry{
 		Ino:  de.Ino,
 		Mode: (uint32(de.Type) << 12),