ipv6: mcast: remove one synchronize_net() barrier in ipv6_mc_down()

As discussed in the past (commit 2d3916f3 ("ipv6: fix skb drops
in igmp6_event_query() and igmp6_event_report()")) I think the
synchronize_net() call in ipv6_mc_down() is not needed.

Under load, synchronize_net() can last between 200 usec and 5 ms.

KASAN seems to agree as well.

Fixes: f185de28 ("mld: add new workqueues for process mld events")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Taehee Yoo <ap420073@gmail.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>