ipv6: Make 'addrconf_rs_timer' send Router Solicitations (and re-arm itself) if Router Advertisements are accepted

As of 026359bc [ipv6: Send ICMPv6 RSes only when RAs are accepted],
Router Solicitations are sent whenever kernel accepts Router
Advertisements on the interface.

However, this logic isn't reflected in 'addrconf_rs_timer'.

The timer fails to issue subsequent RS messages (and fails to re-arm
itself) if forwarding is enabled and the special hybrid mode is
enabled (accept_ra=2).

Fix the condition determining whether next RS should be sent, by using
'ipv6_accept_ra()'.
Reported-by: NAmi Koren <amikoren@yahoo.com>
Signed-off-by: NShmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>