CVE-2024-42265 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.