net/igb: free consumed Tx buffers on demand

net/igb: free consumed Tx buffers on demand

Add support to the e1000 igb driver for the new API to force free
consumed buffers on Tx ring. This API is independent of the tx_rs_thresh
setting. With this API, buffers should be free even if tx_rs_thresh is
not met.

e1000 igb driver does not implement a tx_rs_thresh to free mbufs, it
frees a slot in the ring as needed. However, it could be implemented at
some future date.

Signed-off-by: Billy McFall <bmcfall@redhat.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Keith Wiles <keith.wiles@intel.com>