ipv4: add mc_count to in_device.

This patch add mc_count to struct in_device and updates
increment/decrement/initilaize of this field in IPv4 and in IPv6.

- Also printing the vfs /proc entry (/proc/net/igmp) is adjusted to
use the new mc_count.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>