i40evf: skb->xmit_more support

Eric added support for skb->xmit_more in i40e, this ports that into
i40evf as well.

Support skb->xmit_more in i40evf is straightforward; we need to move
around i40e_maybe_stop_tx() call to correctly test netif_xmit_stopped()
before taking the decision to not kick the NIC.

Change-ID: Idddda6a2e4a7ab335631c91ced51f55b25eb8468
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>