apparmor: cache the are-we-enabled decision

Since we check /sys/kernel/security/ files when deciding whether
apparmor is enabled, and that might not be mounted in the container,
we cannot re-make the decision at apparmor_process_label_set() time.
Luckily we don't have to - just cache the decision made at
lsm_apparmor_drv_init().

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>