MDEV-6318: MariaDB with XtraDB uses times more of IO events

  than with InnoDB plugin
  
  Fix: os0file.h in XtraDB had OS_AIO_N_PENDING_IOS_PER_THREAD 256
  when on InnoDB it is OS_AIO_N_PENDING_IOS_PER_THREAD 32. Changed
  XtraDB also to use 32.

storage/xtradb/include/os0file.h

@@ -151,8 +151,8 @@ log. */
 #define OS_FILE_LOG	256	/* This can be ORed to type */
 /* @} */
 
-#define OS_AIO_N_PENDING_IOS_PER_THREAD 256	/*!< Windows might be able to handle
-more */
+#define OS_AIO_N_PENDING_IOS_PER_THREAD 32	/*!< Win NT does not allow more
+						than 64 */
 
 /** Modes for aio operations @{ */
 #define OS_AIO_NORMAL	21	/*!< Normal asynchronous i/o not for ibuf