gnttab: avoid spurious maptrack handle allocation failures

When no memory is available in the hypervisor, rather than immediately
failing the request try to steal a handle from another vCPU.

Reported-by: George Dunlap <george.dunlap@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -397,7 +397,7 @@ get_maptrack_handle(
     struct vcpu          *curr = current;
     unsigned int          i, head;
     grant_handle_t        handle;
-    struct grant_mapping *new_mt;
+    struct grant_mapping *new_mt = NULL;
 
     handle = __get_maptrack_handle(lgt, curr);
     if ( likely(handle != -1) )
@@ -408,8 +408,13 @@ get_maptrack_handle(
     /*
      * If we've run out of frames, try stealing an entry from another
      * VCPU (in case the guest isn't mapping across its VCPUs evenly).
+     * Also use this path in case we're out of memory, to avoid spurious
+     * failures.
      */
-    if ( nr_maptrack_frames(lgt) >= max_maptrack_frames )
+    if ( nr_maptrack_frames(lgt) < max_maptrack_frames )
+        new_mt = alloc_xenheap_page();
+
+    if ( !new_mt )
     {
         /*
          * Can drop the lock since no other VCPU can be adding a new
@@ -432,12 +437,6 @@ get_maptrack_handle(
         return steal_maptrack_handle(lgt, curr);
     }
 
-    new_mt = alloc_xenheap_page();
-    if ( !new_mt )
-    {
-        spin_unlock(&lgt->maptrack_lock);
-        return -1;
-    }
     clear_page(new_mt);
 
     /*