block: remove padding from io_context on 64bit builds

On 64 bit builds when CONFIG_BLK_CGROUP=n (the default) this removes 8
bytes of padding from structure io_context and drops its size from 72 to
64 bytes, so needing one fewer cachelines and allowing more objects per
slab in it's kmem_cache.
Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>

----
patch against 2.6.33
compiled & test on x86_64 AMDX2
regards
Richard
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/iocontext.h

@@ -49,8 +49,8 @@ struct io_context {
 	/*
 	 * For request batching
 	 */
-	unsigned long last_waited; /* Time last woken after wait for request */
 	int nr_batch_requests;     /* Number of requests left in the batch */
+	unsigned long last_waited; /* Time last woken after wait for request */
 
 	struct radix_tree_root radix_root;
 	struct hlist_head cic_list;