fm10k: Add skb->xmit_more support

fm10k: Add skb->xmit_more support

This change adds support for skb->xmit_more based on the changes that were
made to igb to support the feature.  The main changes are moving up the
check for maybe_stop_tx so that we can check netif_xmit_stopped to determine
if we must write the tail because we can add no further buffers.

Acked-by: Matthew Vick <[email protected]>
Signed-off-by: Alexander Duyck <[email protected]>
Acked-by: Jeff Kirsher <[email protected]>
Signed-off-by: David S. Miller <[email protected]>