btrfs: replace EINVAL with EOPNOTSUPP for dev_replace raid56

To return EOPNOTSUPP is more user friendly than to return EINVAL,
and then user-space tool will show that the dev_replace operation
for raid56 is not currently supported rather than showing that
there is an invalid argument.
Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/dev-replace.c

@@ -313,7 +313,7 @@ int btrfs_dev_replace_start(struct btrfs_root *root,
 
 	if (btrfs_fs_incompat(fs_info, RAID56)) {
 		btrfs_warn(fs_info, "dev_replace cannot yet handle RAID5/RAID6");
-		return -EINVAL;
+		return -EOPNOTSUPP;
 	}
 
 	switch (args->start.cont_reading_from_srcdev_mode) {