ipv6: addrconf: do not block BH in ipv6_chk_addr_and_flags()

rcu_read_lock() is enough here, as inet6_ifa_finish_destroy()
uses kfree_rcu()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>