x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index()

In the Linux kernel, the following vulnerability has been resolved:

x86/hyperv: Avoid using per-cpu output page in hv_apicid_to_vp_index()

hv_apicid_to_vp_index() currently uses the per-cpu hypercall input
and output pages. This function is called when running in VTL2 and
when running in an SEV-SNP CoCo VM with no paravisor. In the former
case, the output page is allocated, but in the latter case it is
not, so the hypervisor stores the output VP index in memory that has
not been allocated by the guest.

Fix this by using the input page for both input and output. The
hypercall has very small input and output, so sharing the same
page for both is straightforward. An alternative fix is to
allocate the per-cpu output page when running in an SEV-SNP CoCo
guest, but this uses significantly more memory, particularly
with larger vCPUs counts.