bonding: add bond_tx_drop() helper

Because bonding stats are usually sum of slave stats, it was
not easy to account for tx drops at bonding layer.

We can use dev->tx_dropped for this, as this counter is later
added to the device stats (in dev_get_stats())

This extends the idea we had in commit ee6377147409a ("bonding: Simplify
the xmit function for modes that use xmit_hash") for bond_3ad_xor_xmit()
to other bonding modes.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mahesh Bandewar <maheshb@google.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@redhat.com>
Acked-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>