[IPV6]: Acquire addrconf_hash_lock for read in addrconf_verify(...)

addrconf_verify(...) only traverse address hash table when
addrconf_hash_lock is held for writing, and it may hold
addrconf_hash_lock for a long time. So I think it's better to acquire
addrconf_hash_lock for reading instead of writing

Signed-off-by: Yan Zheng <yanzheng@21cn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>