apparmor: Fix ref count leak in task_kill

apparmor_task_kill was not putting the task_cred reference tc, or the
cred_label reference tc when dealing with a passed in cred, fix this
by using a single fn exit.

Fixes: 90c436a6 ("apparmor: pass cred through to audit info.")
Signed-off-by: John Johansen <john.johansen@canonical.com>