6lowpan: handle context based source address

Handle context based address when an unspecified address is given.
For other context based address we print a warning and drop the packet
because we don't support it right now.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>