sysctl: adjust XEN_SYSCTL_numainfo behavior

... to match XEN_SYSCTL_cputopoinfo, allowing the caller to get what it
needs (if e.g. it's after the data for just one specific node) with
just one hypercall, without caring about the total number of nodes in
the system.

Suggested-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/sysctl.c
+++ b/xen/common/sysctl.c
@@ -285,15 +285,9 @@ long do_sysctl(XEN_GUEST_HANDLE_PARAM(xe
         {
             xen_sysctl_meminfo_t meminfo = { 0 };
 
-            if ( ni->num_nodes < num_nodes )
-            {
-                ret = -ENOBUFS;
-                i = num_nodes;
-            }
-            else
-                i = 0;
-
-            for ( ; i < num_nodes; i++ )
+            if ( num_nodes > ni->num_nodes )
+                num_nodes = ni->num_nodes;
+            for ( i = 0; i < num_nodes; ++i )
             {
                 static uint32_t distance[MAX_NUMNODES];
 
@@ -335,7 +329,7 @@ long do_sysctl(XEN_GUEST_HANDLE_PARAM(xe
         else
             i = num_nodes;
 
-        if ( (!ret || (ret == -ENOBUFS)) && (ni->num_nodes != i) )
+        if ( !ret && (ni->num_nodes != i) )
         {
             ni->num_nodes = i;
             if ( __copy_field_to_guest(u_sysctl, op,
--- a/xen/include/public/sysctl.h
+++ b/xen/include/public/sysctl.h
@@ -515,10 +515,11 @@ DEFINE_XEN_GUEST_HANDLE(xen_sysctl_memin
  *    non-null)
  *
  * OUT:
- *  - If 'num_nodes' is less than the number Xen needs to write, -ENOBUFS shall
- *    be returned and 'num_nodes' updated to reflect the intended number.
- *  - On success, 'num_nodes' shall indicate the number of entries written, which
- *    may be less than the maximum.
+ *  - If 'num_nodes' is less than the number Xen wants to write but either
+ *    handle is not a NULL one, partial data gets returned and 'num_nodes'
+ *    gets updated to reflect the intended number.
+ *  - Otherwise, 'num_nodes' shall indicate the number of entries written, which
+ *    may be less than the input value.
  */
 
 struct xen_sysctl_numainfo {