ptrace: ptrace_check_attach() should not do s/STOPPED/TRACED/

After "ptrace: Clean transitions between TASK_STOPPED and TRACED"
d79fdd6d, ptrace_check_attach()
should never see a TASK_STOPPED tracee and s/STOPPED/TRACED/ is
no longer legal. Add the warning.

Note: ptrace_check_attach() can be greatly simplified, in particular
it doesn't need tasklist. But I'd prefer another patch for that.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>