Kernel: Remove 1 level of indirection for AnonymousVMObject CoW bitmaps

Instead of keeping AnonymousVMObject::m_cow_map in an OwnPtr<Bitmap>,
just make the Bitmap a regular value member. This increases the size
of the VMObject by 8 bytes, but removes some of the kmalloc/kfree spam
incurred by sys$fork().

Kernel/VM/AnonymousVMObject.h

@@ -147,7 +147,7 @@ private:
     Vector<PurgeablePageRanges*> m_purgeable_ranges;
     size_t m_unused_committed_pages { 0 };
 
-    mutable OwnPtr<Bitmap> m_cow_map;
+    Bitmap m_cow_map;
 
     // We share a pool of committed cow-pages with clones
     RefPtr<CommittedCowPages> m_shared_committed_cow_pages;