fsdax: dax_unshare_iter() should return a valid length

The copy_mc_to_kernel() will return 0 if it executed successfully. Then
the return value should be set to the length it copied.

[[e-mail]: don't mess up `ret', per Matthew]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: d984648e428b ("fsdax,xfs: port unshare to fsdax")
Signed-off-by: Shiyang Ruan <[e-mail]>
Cc: Darrick J. Wong <[e-mail]>
Cc: Alistair Popple <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Dave Chinner <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/dax.c

···
 	if (ret < 0)
 		goto out_unlock;
 
-	ret = copy_mc_to_kernel(daddr, saddr, length);
-	if (ret)
+	if (copy_mc_to_kernel(daddr, saddr, length) == 0)
+		ret = length;
+	else
 		ret = -EIO;
 
 out_unlock: