pipe: change pipemapping cnt/pos to u_int

This matches the case using regular buffers and avoids an ifdef later.

Suggested by:	markj

sys/sys/pipe.h

@@ -74,8 +74,8 @@ struct pipebuf {
  * Information to support direct transfers between processes for pipes.
  */
 struct pipemapping {
-	vm_size_t	cnt;		/* number of chars in buffer */
-	vm_size_t	pos;		/* current position of transfer */
+	u_int		cnt;		/* number of chars in buffer */
+	u_int		pos;		/* current position of transfer */
 	int		npages;		/* number of pages */
 	vm_page_t	ms[PIPENPAGES];	/* pages in source process */
 };