KVM: s390: gaccess: Cleanup access to guest pages

[ Upstream commit bad13799e0305deb258372b7298a86be4c78aaba ]

Introduce a helper function for guest frame access.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20211126164549.7046-4-scgl@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Stable-dep-of: e8061f06185b ("KVM: s390: gaccess: Check if guest address is in memslot")
Signed-off-by: Sasha Levin <sashal@kernel.org>