inet: frags: use INC_STATS_BH in the ipv6 reassembly code

Softirqs are already disabled so no need to do it again, thus let's be
consistent and use the IP6_INC_STATS_BH variant.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>