[AIO] use aio_down()/aio_up() for generic_file_aio_write

Now that we have an async down() primative, make use of it for 
generic_file_aio_write().

Signed-off-by: Benjamin LaHaise <bcrl@linux.intel.com>
diff -purN --exclude=description 20_aio_down/mm/filemap.c 30_aio_write/mm/filemap.c
--- 20_aio_down/mm/filemap.c	2005-08-04 15:56:09.000000000 -0400
+++ 30_aio_write/mm/filemap.c	2005-08-08 17:15:32.000000000 -0400
@@ -2143,10 +2143,12 @@ ssize_t generic_file_aio_write(struct ki
 
 	BUG_ON(iocb->ki_pos != pos);
 
-	down(&inode->i_sem);
+	ret = aio_down(iocb, &inode->i_sem);
+	if (ret)
+		return ret;
 	ret = __generic_file_aio_write_nolock(iocb, &local_iov, 1,
 						&iocb->ki_pos);
-	up(&inode->i_sem);
+	aio_up(iocb, &inode->i_sem);
 
 	if (ret > 0 && ((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
 		ssize_t err;