net: ipv6: Fix crash from double free of fragment

This commit fixes crash caused by double free from message sequence
fragmentation. When double free happens the call stack is:

0  reassembly_cancel
1  handle_fragment_hdr
2  net_ipv6_process_pkt
3  process_data
4  processing_data
5  net_rx
6  process_rx_packet
7  work_q_main
8  _thread_entry

So at first packet is unrefed in reassembly_cancel and then also in
processing_data.

Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>