aio: partial write should not return error code

When an AIO write gets an error after writing some data (eg.  ENOSPC), it
should return the amount written already, not the error.  Just like write()
is supposed to.

This was found by the libaio test suite.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Acked-By: NZach Brown <zach.brown@oracle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/aio.c

@@ -1347,6 +1347,13 @@ static ssize_t aio_rw_vect_retry(struct kiocb *iocb)
 	if ((ret == 0) || (iocb->ki_left == 0))
 		ret = iocb->ki_nbytes - iocb->ki_left;
 
+	/* If we managed to write some out we return that, rather than
+	 * the eventual error. */
+	if (opcode == IOCB_CMD_PWRITEV
+	    && ret < 0 && ret != -EIOCBQUEUED && ret != -EIOCBRETRY
+	    && iocb->ki_nbytes - iocb->ki_left)
+		ret = iocb->ki_nbytes - iocb->ki_left;
+
 	return ret;
 }