6lowpan: move skb_free from error paths in decompression

Currently we ensure that the skb is freed on every error path in IPHC
decompression which makes it easy to introduce skb leaks.  By centralising
the skb_free into the receive function it makes future decompression routines
easier to maintain.  It does come at the expense of ensuring that the skb
passed into the decompression routine must not be copied.

Signed-off-by: Martin Townsend <mtownsend1973@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>