Add __rcu annotations so sparse correctly warns only if "slot" gets
derefenced without using rcu_dereference(). Right now we get warnings
because of the missing annotation:

arch/s390/mm/gmap.c:135:17: warning: incorrect type in assignment (different address spaces)
arch/s390/mm/gmap.c:135:17:    expected void **slot
arch/s390/mm/gmap.c:135:17:    got void [noderef] <asn:4>**
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>