bonding: drop_monitor aware

When packets are dropped in TX path, its better to use kfree_skb()
instead of dev_kfree_skb() to give proper drop_monitor events.

Also move the kfree_skb() call after read_unlock() in bond_alb_xmit()
and bond_xmit_activebackup()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>