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>