s390/mm: mark private defines for vm_fault_t as such

This fixes several sparse warnings for fault.c:

arch/s390/mm/fault.c:336:36: warning: restricted vm_fault_t degrades to integer
arch/s390/mm/fault.c:573:23: warning: incorrect type in assignment (different base types)
arch/s390/mm/fault.c:573:23:    expected restricted vm_fault_t [usertype] fault
arch/s390/mm/fault.c:573:23:    got int

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>