ipv6: add special mode forwarding=2 to send RS while configured as router

Similar to accepting router advertisement, the IPv6 stack does not send router
solicitations if forwarding is enabled.

This patch enables this behavior to be overruled by setting forwarding to the
special value 2.

Signed-off-by: Thomas Graf <tgraf@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>