x86/pod: fix decrease_reservation() clearing of M2P entries

Add the missing index increase in the M2P clearing loop, otherwise the loop
keeps pointlessly setting the same MFN entry repeatedly.  This seems to be
an oversight from the change that introduced support to process high order
pages in one go.

Fixes: 3c352011c0 ("x86/PoD: shorten certain operations on higher order ranges")
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 8e4d495cb8
master date: 2025-12-11 09:50:10 +0100

xen/arch/x86/mm/p2m-pod.c

@@ -655,7 +655,7 @@ decrease_reservation(struct domain *d, gfn_t gfn, unsigned int order)
             }
             p2m_tlb_flush_sync(p2m);
             for ( j = 0; j < n; ++j )
-                set_gpfn_from_mfn(mfn_x(mfn), INVALID_M2P_ENTRY);
+                set_gpfn_from_mfn(mfn_x(mfn) + j, INVALID_M2P_ENTRY);
             p2m_pod_cache_add(p2m, page, cur_order);
 
             ioreq_request_mapcache_invalidate(d);