batman-adv: fix lockdep splat when doing mcast_free

batman-adv: fix lockdep splat when doing mcast_free

While testing, we got something like this:

WARNING: CPU: 0 PID: 238 at net/batman-adv/multicast.c:142 batadv_mcast_mla_tt_retract+0x94/0x205 [batman_adv]()
[...]
Call Trace:
[<ffffffff815fc597>] dump_stack+0x4b/0x64
[<ffffffff810b34dc>] warn_slowpath_common+0xbc/0x120
[<ffffffffa0024ec5>] ? batadv_mcast_mla_tt_retract+0x94/0x205 [batman_adv]
[<ffffffff810b3705>] warn_slowpath_null+0x15/0x20
[<ffffffffa0024ec5>] batadv_mcast_mla_tt_retract+0x94/0x205 [batman_adv]
[<ffffffffa00273fe>] batadv_mcast_free+0x36/0x39 [batman_adv]
[<ffffffffa0020c77>] batadv_mesh_free+0x7d/0x13f [batman_adv]
[<ffffffffa0036a6b>] batadv_softif_free+0x15/0x25 [batman_adv]
[...]

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>