nfs: fix bug about IPv6 address scope checking

The result from ipv6_addr_scope() always not be a single SCOPE,
so we can't use equal to compare the result with IPV6_ADDR_SCOPE_LINKLOCAL
at nfs_sockaddr_match_ipaddr6.

This patch fixs the problem, and lets checking address before scope_id.

Signed-off-by: Mi Jinlong <mijinlong@cn.fujitsu.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>