frag6: Fix fragment reassembly

r337776 started hashing the fragments into buckets for faster lookup.

The hashkey is larger than intended. This results in random stack data being
included in the hashed data, which in turn means that fragments of the same
packet might end up in different buckets, causing the reassembly to fail.

Set the correct size for hashkey.

PR: 231045
Approved by: re (kib)
MFC after: 3 days