idr: document exit conditions on idr_for_each_entry better

And some manual common subexpression elimination which may help the
compiler produce smaller code.

Signed-off-by: George Spelvin <linux@horizon.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/idr.h

@@ -122,11 +122,13 @@ static inline void *idr_find(struct idr *idr, int id)
  * @idp:     idr handle
  * @entry:   the type * to use as cursor
  * @id:      id entry's key
+ *
+ * @entry and @id do not need to be initialized before the loop, and
+ * after normal terminatinon @entry is left with the value NULL.  This
+ * is convenient for a "not found" value.
  */
-#define idr_for_each_entry(idp, entry, id)				\
-	for (id = 0, entry = (typeof(entry))idr_get_next((idp), &(id)); \
-	     entry != NULL;                                             \
-	     ++id, entry = (typeof(entry))idr_get_next((idp), &(id)))
+#define idr_for_each_entry(idp, entry, id)			\
+	for (id = 0; ((entry) = idr_get_next(idp, &(id))) != NULL; ++id)
 
 /*
  * Don't use the following functions.  These exist only to suppress