Revert "drivers/tty: Explicitly pass current to show_stack"

This reverts commit 9f12cea96f.

Mark writes:
	Unfortunately, this patch will result in erroneous stack traces
	on some architectures. Sorry about this; I should have verified
	this more thoroughly before sending the series out.

	Please drop the patch at your earliest convenience.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/sysrq.c

@@ -224,7 +224,7 @@ static void showacpu(void *dummy)
 
 	spin_lock_irqsave(&show_lock, flags);
 	pr_info("CPU%d:\n", smp_processor_id());
-	show_stack(current, NULL);
+	show_stack(NULL, NULL);
 	spin_unlock_irqrestore(&show_lock, flags);
 }