KVM: s390: Fix gmap_link()

KVM: s390: Fix gmap_link()

The slow path of the fault handler ultimately called gmap_link(), which
assumed the fault was a major fault, and blindly called dat_link().

In case of minor faults, things were not always handled properly; in
particular the prefix and vsie marker bits were ignored.

Move dat_link() into gmap.c, renaming it accordingly. Once moved, the
new _gmap_link() function will be able to correctly honour the prefix
and vsie markers.

This will cause spurious unshadows in some uncommon cases.

Fixes: 94fd9b16cc67 ("KVM: s390: KVM page table management functions: lifecycle management")
Fixes: a2c17f9270cc ("KVM: s390: New gmap code")
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>