Bluetooth: L2CAP: copy RX timestamp to new fragments

Copy timestamp too when allocating new skb for received fragment.
Fixes missing RX timestamps with fragmentation.

Fixes: 4d7ea8ee ("Bluetooth: L2CAP: Fix handling fragmented length")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>