ipv6 mroute: Use MRT6_VERSION instead of MRT_VERSION in ip6mr.c.

MRT6_VERSION should be used instead of MRT_VERSION in ip6mr.c.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>