KVM: s390: vsie: Fix check for pre-existing shadow mapping

KVM: s390: vsie: Fix check for pre-existing shadow mapping

When shadowing a nested guest, a check is performed and no shadowing is
attempted if the nested guest is already shadowed.

The existing check was incomplete; fix it by also checking whether the
leaf DAT table entry in the existing shadow gmap has the same protection
as the one specified in the guest DAT entry.

Fixes: e38c884df921 ("KVM: s390: Switch to new gmap")
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>