Bluetooth: 6lowpan: Ensure header compression does not corrupt IPv6 header

If skb is going to multiple destinations, then make sure that we
do not overwrite the common IPv6 headers. So before compressing
the IPv6 headers, we copy the skb and that is then sent to 6LoWPAN
Bluetooth devices.

This is a similar patch as what was done for IEEE 802.154 6LoWPAN
in commit f19f4f95 ("ieee802154: 6lowpan: ensure header compression
does not corrupt ipv6 header")

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>