frag6: handling of overlapping fragments to conform to RFC 8200

While the comment was updated in r350746, the code was not.
RFC8200 says that unless fragment overlaps are exact (same fragment
twice) not only the current fragment but the entire reassembly queue
for this packet must be silently discarded, which we now do if
fragment offset and fragment length do not match.

Obtained from: jtl
MFC after: 3 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D16850