apparmor: avoid per-cpu hold underflow in aa_get_buffer

In the Linux kernel, the following vulnerability has been resolved:

apparmor: avoid per-cpu hold underflow in aa_get_buffer

When aa_get_buffer() pulls from the per-cpu list it unconditionally

decrements cache->hold. If hold reaches 0 while count is still non-zero,

the unsigned decrement wraps to UINT_MAX. This keeps hold non-zero for a

very long time, so aa_put_buffer() never returns buffers to the global

list, which can starve other CPUs and force repeated kmalloc(aa_g_path_max)

allocations.

Guard the decrement so hold never underflows.