batman-adv: fix global protection fault during soft_iface destruction

batadv_mesh_free() schedules some RCU callbacks which need the bat_priv struct
to do their jobs, while free_netdev(), which is called immediately after, is
destroying the private data.

Put an rcu_barrier() in the middle so that free_netdev() is invoked only after
all the callbacks returned.

This bug has been introduced by ab8f433dd39be94e8617cff2dfe9f7eca162eb15
("batman-adv: Move deinitialization of soft-interface to destructor")

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: David S. Miller <davem@davemloft.net>