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 ->fd[fd] in do_dup2() from mispredictions both callers have verified that fd is not greater than ->max_fds; however, misprediction might end up with tofree = fdt->fd[fd]; being speculatively executed. That's wrong for the same reasons why it's wrong in close_fd()/file_close_fd_locked(); the same solution applies - array_index_nospec(fd, fdt->max_fds) could differ from fd only in case of speculative execution on mispredicted path.