Strict checking for link-local address

RFC 4291 requires that link-local addresses are within fe80::/64 even if the
link-local prefix is fe80::/10.

RFC 4291 IP Version 6 Addressing Architecture
  2.5.6.  Link-Local IPv6 Unicast Addresses

     Link-Local addresses are for use on a single link.  Link-Local
     addresses have the following format:

     |   10     |
     |  bits    |         54 bits         |          64 bits           |
     +----------+-------------------------+----------------------------+
     |1111111010|           0             |       interface ID         |
     +----------+-------------------------+----------------------------+