apparmor: fix invalid reference to index variable of iterator line 836

Once the loop on lines 836-853 is complete and exits normally, ent is a
pointer to the dummy list head value.  The derefernces accessible from eg
the goto fail on line 860 or the various goto fail_lock's afterwards thus
seem incorrect.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>