KVM: Disallow fork() and similar games when using a VM

We don't want the meaning of guest userspace changing under our feet.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/kernel/fork.c b/kernel/fork.c
index 314f510..05e0b6f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -393,6 +393,7 @@
 	destroy_context(mm);
 	free_mm(mm);
 }
+EXPORT_SYMBOL_GPL(__mmdrop);
 
 /*
  * Decrement the use count and release all resources for an mm.