kernel: protect the fetch of ->fd[fd] in do_dup2() from mispredictions

In the Linux kernel, the following vulnerability has been resolved: protect the fetch of -fdfd in dodup2 from mispredictions both callers have verified that fd is not greater than -maxfds; however, misprediction might end up with tofree = fdt-fdfd; being speculatively executed. That's wrong for t...