signal: Revert ptrace preempt magic

Upstream commit '53da1d9456fe7f8 fix ptrace slowness' is nothing more
than a bandaid around the ptrace design trainwreck. It's not a
correctness issue, it's merily a cosmetic bandaid.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/signal.c

@@ -2288,16 +2288,8 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 		if (gstop_done && ptrace_reparented(current))
 			do_notify_parent_cldstop(current, false, why);
 
-		/*
-		 * Don't want to allow preemption here, because
-		 * sys_ptrace() needs this task to be inactive.
-		 *
-		 * XXX: implement read_unlock_no_resched().
-		 */
-		preempt_disable();
 		read_unlock(&tasklist_lock);
 		cgroup_enter_frozen();
-		preempt_enable_no_resched();
 		freezable_schedule();
 		cgroup_leave_frozen(true);
 	} else {