apparmor: document the buffer hold, add an overflow guard

The buffer hold is a measure of contention, but it is tracked per cpu
where the lock is a globabl resource. On some systems (eg. real time)
there is no guarantee that the code will be on the same cpu pre, and
post spinlock acquisition, nor that the buffer will be put back to
the same percpu cache when we are done with it.

Because of this the hold value can move asynchronous to the buffers on
the cache, meaning it is possible to underflow, and potentially in really
pathelogical cases overflow.

Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

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