router: skip RA and wait for LINK-LOCAL to be assigned

router: skip RA and wait for LINK-LOCAL to be assigned

This fix a specific and corner case when the following error and similar
is printed in the log:

Failed to send to ff02::1%br-lan (Address not available)

The cause for this was tracked down to the lack of the interface of a
configured LINK-LOCAL IPV6 address resulting in odhcpd_send() always
failing.

A LINK-LOCAL IPV6 address is assigned only after the interface has
carrier and is set to IFF_RUNNING and require some time for the address
to be assigned due to DAD logic.

In the case where an interface was just UP, odhcpd RA may fail since the
LINK-LOCAL IPV6 address still needs to be assigned as it still need to
be "trained". From the kernel view this is flagged in the IPV6 interface
address with the flag IFA_F_TENTATIVE, that means the address still
needs to be checked and follow DAD process.

This is only a transient problem and the DAD process is required only
once till the interface is not set DOWN.

To handle this, add some check to verify if the address has to be
checked and add an additional bool to flag if the interface have a
LINK-LOCAL assigned.

Skip sending RA if the interface still doesn't have finished the DAD
process and retry at the next RA.
A notice log is added to track this special case to track problematic
case and even more corner case.

Logic to check if interface have LINK-LOCAL are:
- When interface is setup, on scanning for the interface ipv6 address
  check if at least one address is NOT in IFA_F_TENTATIVE state.
- With interface already up but with still no LINK-LOCAL react on the
  RTM_NEWADDR event and set LINK-LOCAL if the addrs added by the event
  is a LINK-LOCAL reflecting that the interface finally ended the DAD
  process and have a correct address.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Acked-by: Hans Dedecker <dedeckeh@gmail.com>