[NTOS:MM] Acquire MmSystemLoadLock in MiFindInitializationCode. CORE-16449

We might otherwise free the INIT section for an image that's in the
process of being loaded by Mm.

ntoskrnl/mm/ARM3/sysldr.c

@@ -1466,8 +1466,13 @@ MiFindInitializationCode(OUT PVOID *StartVa,
     /* Assume failure */
     *StartVa = NULL;
 
-    /* Enter a critical region while we loop the list */
+    /* Acquire the necessary lock while we loop the list */
     KeEnterCriticalRegion();
+    KeWaitForSingleObject(&MmSystemLoadLock,
+                          WrVirtualMemory,
+                          KernelMode,
+                          FALSE,
+                          NULL);
 
     /* Loop all loaded modules */
     NextEntry = PsLoadedModuleList.Flink;
@@ -1615,7 +1620,8 @@ MiFindInitializationCode(OUT PVOID *StartVa,
         NextEntry = NextEntry->Flink;
     }
 
-    /* Leave the critical region and return */
+    /* Release the lock and return */
+    KeReleaseMutant(&MmSystemLoadLock, 1, FALSE, FALSE);
     KeLeaveCriticalRegion();
 }