KVM: s390: pv: fix race when making a page secure

KVM: s390: pv: fix race when making a page secure

Holding the pte lock for the page that is being converted to secure is
needed to avoid races. A previous commit removed the locking, which
caused issues. Fix by locking the pte again.

Fixes: 5cbe24350b7d ("KVM: s390: move pv gmap functions into kvm")
Reported-by: David Hildenbrand <david@redhat.com>
Tested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
[david@redhat.com: replace use of get_locked_pte() with folio_walk_start()]
Link: https://lore.kernel.org/r/20250312184912.269414-2-imbrenda@linux.ibm.com
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20250312184912.269414-2-imbrenda@linux.ibm.com>