[IPV6] MROUTE: Adjust IPV6 multicast routing module to use mroute6 header declarations.

- This patch adjusts IPv6 multicast routing module, net/ipv6/ip6mr.c,
to use mroute6 header definitions instead of mroute.
 (MFC6_LINES instead of MFC_LINES, MAXMIFS instead of MAXVIFS, mifi_t
instead of vifi_t.)

- In addition, inclusion of some headers was removed as it is not needed.

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