ixgbe: Don't priority tag control frames in DCB mode

Certain types of control packets (LLDP, LACP, etc.) are not supposed to have a
priority tag or vlan tag inserted.  Ixgbe driver is currently priority
tagging everything (if packet is not on a VLAN interface).

This patch modifies DCB mode, so that packets marked with skb priority
TC_PRIO_CONTROL are not priority tagged.  It also transmits these packets on
the highest priority traffic class.
Programs (like dcbd) can set the skb priority using a socket option.  Or, a tc
filter can be configured to set the priority value. Using the value
TC_PRIO_CONTROL (7) has the benefit that it is already defined in the kernel,
and the bonding LACP code already sets the skb->priority field to this value.

Signed-off-by: Lucy Liu <lucy.liu@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>