ieee802154: 6lowpan: fix memory leak

Looking at current situation of memory management in 6lowpan receive
function I detected some invalid handling. After calling
lowpan_invoke_rx_handlers we will do a kfree_skb and then NET_RX_DROP on
error handling. We don't do this before, also on
skb_share_check/skb_unshare which might manipulate the reference
counters.

After running some 'grep -r "dev_add_pack" net/' to look how others
packet-layer receive callbacks works I detected that every subsystem do
a kfree_skb, then NET_RX_DROP without calling skb functions which
might manipulate the skb reference counters. This is the reason why we
should do the same here like all others subsystems. I didn't find any
documentation how the packet-layer receive callbacks handle NET_RX_DROP
return values either.

This patch will add a kfree_skb, then NET_RX_DROP handling for the
"trivial checks", in case of skb_share_check/skb_unshare the kfree_skb
call will be done inside these functions.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>