[PATCH] dax: fix regression in dax_writeback_mapping_range()

[Ross Zwisler]

commit 354ae7432ee8 ("dax: add tracepoints to dax_writeback_mapping_range()")
in the -next tree, which appears in next-20170310, inadvertently changed
dax_writeback_mapping_range() so that it could end up returning a positive
value: the number of bytes flushed, as returned by dax_writeback_one().
This was incorrect. This function either needs to return a negative error
value, or zero on success.

This change was causing xfstest failures, as reported by Xiong:

https://lkml.org/lkml/2017/3/13/1220

With this fix applied to next-20170310, all the test failures reported by
Xiong (generic/075 generic/112 generic/127 generic/231 generic/263) are
resolved.

Reported-by: Xiong Zhou <xzhou@xxxxxxxxxx>
Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
---
 fs/dax.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index 1861ef0..60688c7 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -907,7 +907,7 @@ int dax_writeback_mapping_range(struct address_space *mapping,
 	}
 out:
 	trace_dax_writeback_range_done(inode, start_index, end_index);
-	return ret;
+	return (ret < 0 ? ret : 0);
 }
 EXPORT_SYMBOL_GPL(dax_writeback_mapping_range);
 
-- 
2.9.3