ipv6: protect for_each_sk_fl_rcu in mem_check with rcu_read_lock_bh

Fixes a suspicious rcu derference warning.

Cc: Florent Fourcot <florent.fourcot@enst-bretagne.fr>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>