selinux: fix SEGV during switch-root if SELinux policy loaded
If you've got SELinux policy loaded, label_hnd is your labeling handle.
When systemd is shutting down, we free that handle via mac_selinux_finish().
But: switch_root() calls mkdir_p_label(), which tries to look up a label
using that freed handle, and so we get a bunch of garbage and eventually
SEGV in libselinux.
(This doesn't happen in the switch-root from initramfs to real root because
there's no SELinux policy loaded in initramfs, so label_hnd is NULL and we
never attempt any lookups.)
So: make sure that mac_selinux_finish() actually sets label_hnd to NULL, so
nobody tries to use it after it becomes invalid.
https://bugzilla.redhat.com/show_bug.cgi?id=1185604