bonding: reset queue mapping prior to transmission to physical device (v5)

The bonding driver is multiqueue enabled, in which each queue represents a slave
to enable optional steering of output frames to given slaves against the default
output policy.  However, it needs to reset the skb->queue_mapping prior to
queuing to the physical device or the physical slave (if it is multiqueue) could
wind up transmitting on an unintended tx queue

Change Notes:
v2) Based on first pass review, updated the patch to restore the origional queue
mapping that was found in bond_select_queue, rather than simply resetting to
zero.  This preserves the value of queue_mapping when it was set on receive in
the forwarding case which is desireable.

v3) Fixed spelling an casting error in skb->cb

v4) fixed to store raw queue_mapping to avoid double decrement

v5) Eric D requested that ->cb access be wrapped in a macro.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>