direct-io: fix AIO regression

The direct-io.c rewrite to use the iov_iter infrastructure stopped updating
the size field in struct dio_submit, and thus rendered the check for
allowing asynchronous completions to always return false.  Fix this by
comparing it to the count of bytes in the iov_iter instead.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Tim Chen <tim.c.chen@linux.intel.com>
Tested-by: Tim Chen <tim.c.chen@linux.intel.com>

fs/direct-io.c

@@ -71,7 +71,6 @@ struct dio_submit {
 					   been performed at the start of a
 					   write */
 	int pages_in_io;		/* approximate total IO pages */
-	size_t	size;			/* total request size (doesn't change)*/
 	sector_t block_in_file;		/* Current offset into the underlying
 					   file in dio_block units. */
 	unsigned blocks_available;	/* At block_in_file.  changes */
@@ -1104,7 +1103,8 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	unsigned blkbits = i_blkbits;
 	unsigned blocksize_mask = (1 << blkbits) - 1;
 	ssize_t retval = -EINVAL;
-	loff_t end = offset + iov_iter_count(iter);
+	size_t count = iov_iter_count(iter);
+	loff_t end = offset + count;
 	struct dio *dio;
 	struct dio_submit sdio = { 0, };
 	struct buffer_head map_bh = { 0, };
@@ -1287,10 +1287,9 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	 */
 	BUG_ON(retval == -EIOCBQUEUED);
 	if (dio->is_async && retval == 0 && dio->result &&
-	    ((rw == READ) || (dio->result == sdio.size)))
+	    (rw == READ || dio->result == count))
 		retval = -EIOCBQUEUED;
-
-	if (retval != -EIOCBQUEUED)
+	else
 		dio_await_completion(dio);
 
 	if (drop_refcount(dio) == 0) {