apparmor: don't try to replace stale label in ptraceme check

begin_current_label_crit_section() must run in sleepable context because
when label_is_stale() is true, aa_replace_current_label() runs, which uses
prepare_creds(), which can sleep.

Until now, the ptraceme access check (which runs with tasklist_lock held)
violated this rule.

Fixes: b2d09ae4 ("apparmor: move ptrace checks to using labels")
Reported-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Git-commit: ca3fde5214e1d24f78269b337d3f22afd6bf445e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: I3d750f0e457a1755b2fa37aea14df2221c3aa2fc
Signed-off-by: Jiangjiang Shen <jiangjia@codeaurora.org>