s390/kasan: avoid user access code instrumentation

Kasan instrumentation adds "store" check for variables marked as
modified by inline assembly. With user pointers containing addresses
from another address space this produces false positives.

static inline unsigned long clear_user_xc(void __user *to, ...)
{
	asm volatile(
	...
	: "+a" (to) ...

User space access functions are wrapped by manually instrumented
functions in kasan common code, which should be sufficient to catch
errors. So, we just disable uaccess.o instrumentation altogether.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>