[IPV6] MROUTE: Use skb_tail_pointer(skb) instead of skb->tail.
[IPV6] MROUTE: Use skb_tail_pointer(skb) instead of skb->tail.

This bug resulted in compilation error on 64bit machines.
Pointed out by Rami Rosen <roszenrami@gmail.com>.

Signed-off-by: YOSHIFUJI Hideaki <948b80da4254122ea1e8973dd9930d46b7654cf3@linux-ipv6.org>

KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs