Add IPv4 fragments reassembling to NAT64LSN.

NAT64LSN requires the presence of upper level protocol header
in a IPv4 datagram to find corresponding state to make translation.
Now it will be handled automatically by nat64lsn instance.

Reviewed by: melifaro
Obtained from: Yandex LLC
MFC after: 1 week
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D26758