[NET]: Use existing device list walker for /proc/dev_mcast.

The seq_file_operations' dev_mc_seq_xxx callbacks do the same thing as
the dev_seq_xxx ones do, but skip the SEQ_START_TOKEN.

So use the existing exported dev_seq_xxx calls and handle the
SEQ_START_TOKEN in the dev_mc_seq_show().

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>