dax: set did_zero to true when zeroing successfully

It is unnecessary to check and set did_zero value in while() loop
in dax_zero_iter(), we can set did_zero to true only when zeroing
successfully at last.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/dax.c

@@ -1088,10 +1088,10 @@ static s64 dax_zero_iter(struct iomap_iter *iter, bool *did_zero)
 		pos += size;
 		length -= size;
 		written += size;
-		if (did_zero)
-			*did_zero = true;
 	} while (length > 0);
 
+	if (did_zero)
+		*did_zero = true;
 	return written;
 }