memory: fix XENMEM_exchange error handling

assign_pages() can fail due to the domain getting killed in parallel,
which should not result in a hypervisor crash.

Also delete a redundant put_gfn() - all relevant paths leading to the
"fail" label already do this (and there are also paths where it was
plain wrong). All of the put_gfn()-s got introduced by 51032ca058
("Modify naming of queries into the p2m"), including the otherwise
unneeded initializer for k (with even a kind of misleading comment -
the compiler warning could actually have served as a hint that the use
is wrong).

This is XSA-159.

Reported-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

--- a/xen/common/memory.c
+++ b/xen/common/memory.c
@@ -334,7 +334,7 @@ static long memory_exchange(XEN_GUEST_HA
     PAGE_LIST_HEAD(out_chunk_list);
     unsigned long in_chunk_order, out_chunk_order;
     xen_pfn_t     gpfn, gmfn, mfn;
-    unsigned long i, j, k = 0; /* gcc ... */
+    unsigned long i, j, k;
     unsigned int  memflags = 0;
     long          rc = 0;
     struct domain *d;
@@ -572,11 +572,12 @@ static long memory_exchange(XEN_GUEST_HA
  fail:
     /* Reassign any input pages we managed to steal. */
     while ( (page = page_list_remove_head(&in_chunk_list)) )
-    {
-        put_gfn(d, gmfn + k--);
         if ( assign_pages(d, page, 0, MEMF_no_refcount) )
-            BUG();
-    }
+        {
+            BUG_ON(!d->is_dying);
+            if ( test_and_clear_bit(_PGC_allocated, &page->count_info) )
+                put_page(page);
+        }
 
  dying:
     rcu_unlock_domain(d);