batman-adv: Decrease hardif refcnt on fragmentation send error

An error before the hardif is found has to free the skb. But every error
after that has to free the skb + put the hard interface.

Fixes: 8def0be8 ("batman-adv: Consume skb in batadv_frag_send_packet")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>