ut_2_power_up(): Add __attribute__((const)), because otherwise this function

is repeatedly called in buf_flush_free_margin() due to the definitions
of BUF_READ_AHEAD_AREA and other macros starting with BUF_READ_AHEAD_.

include/ut0ut.h

@@ -119,7 +119,8 @@ ulint
 ut_2_power_up(
 /*==========*/
 			/* out: first power of 2 which is >= n */
-	ulint	n);	/* in: number != 0 */
+	ulint	n)	/* in: number != 0 */
+	__attribute__((const));
 /****************************************************************
 Sort function for ulint arrays. */