s390/mm: define private VM_FAULT_* reasons from top bits

The current definition already collapse with the generic definition of
vm_fault_reason.  Move the private definitions to allocate bits from the
top of uint so they won't collapse anymore.

Signed-off-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20230205231704.909536-4-peterx@redhat.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>