iomap: fix a regression for partial write errors

When write* wrote some data it should return the amount of written data
and not the error code that caused it to stop.  Fix a recent regression
in iomap_file_buffered_write that caused it to return the errno instead.

Fixes: 219580ee ("iomap: update ki_pos in iomap_file_buffered_write")
Reported-by: kernel test robot <oliver.sang@intel.com>
Reported-by: Cyril Hrubis <chrubis@suse.cz>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.harjani@gmail.com>

fs/iomap/buffered-io.c

@@ -872,7 +872,7 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
 	while ((ret = iomap_iter(&iter, ops)) > 0)
 		iter.processed = iomap_write_iter(&iter, i);
 
-	if (unlikely(ret < 0))
+	if (unlikely(iter.pos == iocb->ki_pos))
 		return ret;
 	ret = iter.pos - iocb->ki_pos;
 	iocb->ki_pos += ret;