s390: get_user_pages_fast() might sleep

Let's annotate it correctly, so we directly get a warning if
we ever were to use it in atomic/preempt_disable/spinlock environment.

Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>