batman-adv: Return non-const ptr in batadv_getlink_net

The returned net_namespace of batadv_getlink_net may be used with functions
that potentially modify the struct. Thus it must return the pointer as
non-const like rtnl_link_ops::get_link_net does.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>