apparmor: Switch to GFP_KERNEL where possible

After removing preempt_disable() from get_buffers() it is possible to
replace a few GFP_ATOMIC allocations with GFP_KERNEL.

Replace GFP_ATOMIC allocations with GFP_KERNEL where the context looks
to bee preepmtible.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: John Johansen <john.johansen@canonical.com>