batman-adv: Fix double-put of vlan object

Each batadv_tt_local_entry hold a single reference to a
batadv_softif_vlan.  In case a new entry cannot be added to the hash
table, the error path puts the reference, but the reference will also
now be dropped by batadv_tt_local_entry_release().

Fixes: a33d970d

 ("batman-adv: Fix reference counting of vlan object for tt_local_entry")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>