net: ipv6_fragment: remove special handling of 1st fragment

The special handling of the 1st fragment in unnecessary, since it will
be correctly handled even without it. Moreover it causes some corner
cases, like a single packet with a fragment header (M=0), to be
incorrectly handled since the reassembly code is skipped.

Remove the special handling of the 1st fragment to fix these problems.

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