nfp: do simple XDP TX buffer recycling

On the RX path we follow the "drop if allocation of replacement
buffer fails" rule.  With XDP we extended that to the TX action,
so if XDP prog returned TX but allocation of replacement RX buffer
failed, we will drop the packet.

To improve our XDP TX performance extend the idea of rings being
always full to XDP TX rings.  Pre-fill the XDP TX rings with RX
buffers, and when XDP prog returns TX action swap the RX buffer
with the next buffer from the TX ring.

XDP TX complete will no longer free the buffers but let them
sit on the TX ring and wait for swap with RX buffer, instead.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>