Fix last incorrect usage of FFI::Pointer#write_string

Author: eregon

src/main/ruby/truffleruby/core/io.rb

@@ -1560,7 +1560,7 @@ def ioctl(command, arg = 0)
       # is still necessary.
       buffer_size = 4096 if buffer_size < 4096
       buffer = Truffle::FFI::MemoryPointer.new(buffer_size)
-      buffer.write_string(arg, arg.bytesize)
+      buffer.write_bytes(arg)
       real_arg = buffer.address
     else
       real_arg = Primitive.rb_to_int(arg)