Instead of directly using the result of dirfd() in the unlinkat() call,
check whether the returned file descriptor is actually valid. That
doesn't really change the logic as the unlinkat() would fail with
invalid descriptor anyway, but this is cleaner and will report the right
error returned directly by dirfd() instead of EBADF from unlinkat().
(cherry picked from commit 59f4fdebc0)
e08d3a7932