net: procfs: add seq_puts() statement for dev_mcast

Add seq_puts() statement for dev_mcast, make it more readable.
As also, keep vertical alignment for {dev, ptype, dev_mcast} that
under /proc/net.

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>