When RA packet with prefix option is received during processing DAD for linklocal
address, Linux initiates DAD for global address derived from the received prefix.
It can be succeeded even if a duplicated linklocal address is detected.

RFC4862 5.4.5, which describes the behaviour on DAD failure, says;

   If the address is a link-local address formed from an interface
   identifier based on the hardware address, which is supposed to be
   uniquely assigned (e.g., EUI-64 for an Ethernet interface), IP
   operation on the interface SHOULD be disabled.  By disabling IP
   operation, the node will then:

   -  not send any IP packets from the interface,

   -  silently drop any IP packets received on the interface, and

   -  not forward any IP packets to the interface (when acting as a
      router or processing a packet with a Routing header).

This problem was observed by testing with beta version of TAHI test suite (v4.0.0b2)
- Stateless Address Autoconfiguration test #3, #5, #14, and #15 force dad for linklocal
to be failed and send RA to the host, then check if the host doesn't respond to
DAD NS with respect to its global address. However, 2.6.26-rc2 send DAD NA in response
to the DAD NS so the test scenarios were failed.

This patch fixes the problem by deferring DAD initiation for global address until
DAD for linklocal address is completed. Now the failed test scenarios noted above
are all passed.

Thank you,
-toyo