postcopy: Synchronize usage of the balloon inhibitor

While the qemu_balloon_inhibit() interface appears rather general purpose,
postcopy uses it in a last-caller-wins approach with no guarantee of balanced
inhibits and de-inhibits.  Wrap postcopy's usage of the inhibitor to give it
one vote overall, using the same last-caller-wins approach as previously
implemented at the balloon level.

Fixes: 01ccbec ("balloon: Allow multiple inhibit users")
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

migration/postcopy-ram.c

@@ -509,6 +509,20 @@ int postcopy_ram_incoming_init(MigrationIncomingState *mis)
     return 0;
 }
 
+/*
+ * Manage a single vote to the QEMU balloon inhibitor for all postcopy usage,
+ * last caller wins.
+ */
+static void postcopy_balloon_inhibit(bool state)
+{
+    static bool cur_state = false;
+
+    if (state != cur_state) {
+        qemu_balloon_inhibit(state);
+        cur_state = state;
+    }
+}
+
 /*
  * At the end of a migration where postcopy_ram_incoming_init was called.
  */
@@ -539,7 +553,7 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
         mis->have_fault_thread = false;
     }
 
-    qemu_balloon_inhibit(false);
+    postcopy_balloon_inhibit(false);
 
     if (enable_mlock) {
         if (os_mlock() < 0) {
@@ -1107,7 +1121,7 @@ int postcopy_ram_enable_notify(MigrationIncomingState *mis)
      * Ballooning can mark pages as absent while we're postcopying
      * that would cause false userfaults.
      */
-    qemu_balloon_inhibit(true);
+    postcopy_balloon_inhibit(true);
 
     trace_postcopy_ram_enable_notify();