Kernel: Preserve internal state in cloned PurgeableVMObjects

When cloning a purgeable memory region (which happens on fork),
we need to preserve the "was purged" and "volatile" state of the
original region, or they will always appear as non-volatile and
unpurged regions in the child process.

Fixes #3374.

Kernel/VM/PurgeableVMObject.cpp

@@ -42,6 +42,8 @@ PurgeableVMObject::PurgeableVMObject(size_t size)
 
 PurgeableVMObject::PurgeableVMObject(const PurgeableVMObject& other)
     : AnonymousVMObject(other)
+    , m_was_purged(other.m_was_purged)
+    , m_volatile(other.m_volatile)
 {
 }