ipv6 sit: Avoid extra need for compat layer in PRL management.

We've introduced extra need of compat layer for ip_tunnel_prl{}
for PRL (Potential Router List) management.  Though compat_ioctl
is still missing in ipv4/ipv6, let's make the interface more
straight-forward and eliminate extra need for nasty compat layer
anyway since the interface is new for 2.6.26.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>