[LMC]: lmc_main wants to use skb_tailroom

At that point it is equivalent to what was being used, skb->end - skb->data,
and the need is clearly the one skb_tailroom satisfies.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>