x86/sysctl: don't clobber memory if NCAPINTS > ARRAY_SIZE(pi->hw_cap)

There is no current problem, as both NCAPINTS and pi->hw_cap are 8 entries,
but the limit should be calculated appropriately so as to avoid hypervisor
stack corruption if the two do get out of sync.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit c373b912e7)

xen/arch/x86/sysctl.c

@@ -75,7 +75,8 @@ long cpu_down_helper(void *data)
 
 void arch_do_physinfo(xen_sysctl_physinfo_t *pi)
 {
-    memcpy(pi->hw_cap, boot_cpu_data.x86_capability, NCAPINTS*4);
+    memcpy(pi->hw_cap, boot_cpu_data.x86_capability,
+           min(sizeof(pi->hw_cap), sizeof(boot_cpu_data.x86_capability)));
     if ( hvm_enabled )
         pi->capabilities |= XEN_SYSCTL_PHYSCAP_hvm;
     if ( iommu_enabled )