apparmor: remove the "task" arg from may_change_ptraced_domain()

Unless task == current ptrace_parent(task) is not safe even under
rcu_read_lock() and most of the current users are not right.

So may_change_ptraced_domain(task) looks wrong as well. However it
is always called with task == current so the code is actually fine.
Remove this argument to make this fact clear.

Note: perhaps we should simply kill ptrace_parent(), it buys almost
nothing. And it is obviously racy, perhaps this should be fixed.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>