Bluetooth: 6lowpan: fix skb_unshare behaviour

This patch reverts commit:

a7807d73 ("Bluetooth: 6lowpan: Avoid memory leak if memory allocation
fails")

which was wrong suggested by Alexander Aring. The function skb_unshare
run also kfree_skb on failure.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org # 3.18.x