[NET]: move dev_mc_discard from dev_mcast.c to dev.c

Because this function is only called by unregister_netdevice,
this moving could make this non-global function static,
and also remove its declaration in netdevice.h;

Any further, function __dev_addr_discard is also just called by
dev_mc_discard and dev_unicast_discard, keeping this two functions
both in one c file could make __dev_addr_discard also static
and remove its declaration in netdevice.h;

Futhermore, the sequential call to dev_unicast_discard and then
dev_mc_discard in unregister_netdevice have a similar mechanism that:
(netif_tx_lock_bh / __dev_addr_discard / netif_tx_unlock_bh),
they should merged into one to eliminate duplicates in acquiring and
releasing the dev->_xmit_lock, this would be done in my following patch.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>