Bluetooth: use kfree_skb() instead of kfree()

sk_buffs should be freed using kfree_skb().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>