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

This reverts commit 9f12cea9

.

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>