The reassembly buffer only needs (and stores) the headers *before* the
fragment header (called per-fragment headers in RFC 8200, section 4.5).
Currently, when a subsequent IPv6 fragment is received before the first
fragment the fragment header is however not removed. With this fix it
does.
The IPv6 (extension) headers of the first fragment received are re-used
for the reassembled packet, so when receiving a subsequent packet we
need to distinguish, if we just want to release the payload or all of
the packet after the packet data was added to the reassembly buffer.
