Since it can be changed via sysfs, we need to make a copy.  This most
generic way of doing this is to keep a flag so we know when to free it.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Cc: Andrew Gallatin <gallatin@myri.com>
Cc: Brice Goglin <brice@myri.com>
Cc: netdev@vger.kernel.org