VMCache: Factor out a _FreePageRange method.

The code in the Resize and Rebase methods was identical except for the
iterator.

Change-Id: I9f6b3c2c09af0c26778215bd627fed030c4d46f1
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2835
Reviewed-by: waddlesplash <[email protected]>

headers/private/kernel/vm/VMCache.h

@@ -215,6 +215,9 @@ private:
 			void				_MergeWithOnlyConsumer();
 			void				_RemoveConsumer(VMCache* consumer);
 
+			bool				_FreePageRange(VMCachePagesTree::Iterator it,
+									page_num_t* toPage);
+
 private:
 			int32				fRefCount;
 			mutex				fLock;