s390: current_stack_pointer shouldn't be a function

s390 defines current_stack_pointer as function while all other
architectures use 'register unsigned long asm("<stackptr reg>").

This make codes like the following from check_stack_object() fail:

	if (IS_ENABLED(CONFIG_STACK_GROWSUP)) {
		if ((void *)current_stack_pointer < obj + len)
			return BAD_STACK;
	} else {
		if (obj < (void *)current_stack_pointer)
			return BAD_STACK;
	}

because this would compare the address of current_stack_pointer() and
not the stackpointer value.

Reported-by: Karsten Graul <kgraul@linux.ibm.com>
Fixes: 2792d84e ("usercopy: Check valid lifetime via stack depth")
Cc: Kees Cook <keescook@chromium.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>