apparmor: use a locallock instead preempt_disable()

get_buffers() disables preemption which acts as a lock for the per-CPU
variable. Since we can't disable preemption here on RT, a local_lock is
lock is used in order to remain on the same CPU and not to have more
than one user within the critical section.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>