net: ipv6_fragment: check the length also for the first fragment

Currently the requirement of the length being a multiple of 8 is not
tested for the first fragment, since the first fragment takes a
different path due to the goto.

Move the test earlier in the process, so that it is performed on all
fragments, including the first one.

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