x86/xen: use new hypercall functions instead of hypercall page

commit b1c2cb86 upstream.

Call the Xen hypervisor via the new xen_hypercall_func static-call
instead of the hypercall page.

This is part of XSA-466 / CVE-2024-53241.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Co-developed-by: Peter Zijlstra <peterz@infradead.org>
Co-developed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>