apparmor spam log with warnings on Preempt kernels

    The apparmor/dbus support needs to allocate buffers in
    atomic context (i.e: holding a spinlock) since that is
    not possible, it declares a static per cpu array of
    buffers and has accessor macros to get and put buffers.

    Since the buffer array is a per cpu variable, it can
    only be concurrently accessed by the same cpu and this
    can only happen if the kernel is preempted.

    So the get_buffers() macro disables preemption with
    preempt_disable() so the buffer can be accessed safely.

    Grabbing a spinlock also makes the kernel to disable
    preemption so a raw __get_buffers() function can be used
    in this case that does not call preempt_disable().

    The raw __get_buffers() function was called from file_path_perm()
    since a spinlock was held by the calling revalidate_tty() function.

    But this is not the only place where file_path_perm() is called,
    it is also called by match_file() which is not in atomic context
    and thus doesn't disable preemption before so the __get_buffers()
    macro was complaining with a WARN_ON(preempt_count() <= 0) and
    spamming the console constantly.

    This patch fix the issue by always calling {get,put}_buffers() since
    preempt_{disable,enable}() functions are nestable.