nfs: fix bug about IPv6 address scope checking

The result from ipv6_addr_scope() is a set of flags, not a single value,
so we can't just compare the result with  IPV6_ADDR_SCOPE_LINKLOCAL.

This patch fixs the problem, and checks for unequal addresses before
scope_id.

Signed-off-by: Mi Jinlong <mijinlong@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>