net: ipv6_fragment: improve reassembly condition

Currently net_ipv6_handle_fragment_hdr() performs 2 distinct tests: it
checks the M-bit of the most recent fragment to decide if we can proceed
with the reassembly. Then it performs some sanity checks which can lead
to dropping the whole packet if not successful.

The test on the M-bit assumes that fragments arrive in order. But this
will fail if packets arrive out-of-order, since the last fragment can
arrive before some other fragments. In that case, we proceed with the
reassembly but it will fail because not all the fragments have been
received.

We need a more complete check before proceeding with the reassembly:
- We received the first fragment (offset = 0)
- All intermediate fragments are contiguous
- The More bit of the last fragment is 0

Since these conditions can also detect a malformed fragmented packet, we
can replace the existing sanity check that is performed before
reassembly. As a bonus, we can now detect and rejected overlapping
fragments, since this can have some security issues (see RFC 5722).

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>