ipv6 anycast: Convert ipv6_sk_ac_lock to spinlock.

Since all users are write-lock, it does not make sense to use
rwlock here.  Use simple spinlock.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>