Bluetooth: 6lowpan: Memory leak as the skb is not freed

The earlier multicast commit 36b3dd25 ("Bluetooth: 6lowpan:
Ensure header compression does not corrupt IPv6 header") lost one
skb free which then caused memory leak.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>