x86/xen: Don't register Xen IPIs when they aren't going to be used

In the case where xen_have_vector_callback is false, we still register
the IPI vectors in xen_smp_intr_init() for the secondary CPUs even
though they aren't going to be used. Stop doing that.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210106153958.584169-5-dwmw2@infradead.org


Signed-off-by: Juergen Gross <jgross@suse.com>