xfrm interface: ifname may be wrong in logs

The ifname is copied when the interface is created, but is never updated
later. In fact, this property is used only in one error message, where the
netdevice pointer is available, thus let's use it.

Fixes: f203b76d

 ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>