vm: add a copy-on-write ref count to vm_page

include/kernel/vm.h

@@ -1,6 +1,7 @@
 #ifndef KERNEL_VM_H_
 #define KERNEL_VM_H_
 
+#include <kernel/atomic.h>
 #include <kernel/bitmap.h>
 #include <kernel/btree.h>
 #include <kernel/locks.h>
@@ -221,6 +222,9 @@ struct vm_page {
 	};
 
 	union {
+		/* how many vm_areas reference this vm_page. if >0, the page is
+		 * subject to copy-on-write. */
+		atomic_t p_cow_ref;
 		uint32_t p_priv2;
 	};