KVM: s390: vsie: stop messing with page refcount

Let's stop messing with the page refcount, and use a flag that is set /
cleared atomically to remember whether a vsie page is currently in use.

Note that we could use a page flag, or a lower bit of the scb_gpa. Let's
keep it simple for now, we have sufficient space.

While at it, stop passing "struct kvm *" to put_vsie_page(), it's
unused.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Tested-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Message-ID: <20250107154344.1003072-4-david@redhat.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>