KVM: s390: Add gmap_helper_set_unused()

KVM: s390: Add gmap_helper_set_unused()

Add gmap_helper_set_unused() to mark userspace ptes as unused.

Core mm code will use that information to discard unused pages instead
of attempting to swap them.

Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Tested-by: Nico Boehr <nrb@linux.ibm.com>
Acked-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>