SELinux: do not check open perms if they are not known to policy

When I introduced open perms policy didn't understand them and I
implemented them as a policycap.  When I added the checking of open perm
to truncate I forgot to conditionalize it on the userspace defined
policy capability.  Running an old policy with a new kernel will not
check open on open(2) but will check it on truncate.  Conditionalize the
truncate check the same as the open check.

Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: stable@vger.kernel.org # 3.4.x
Signed-off-by: James Morris <james.l.morris@oracle.com>