i40e: Invoke softirqs after napi_reschedule

The following message is logged from time to time when using i40e:
NOHZ: local_softirq_pending 08

i40e may schedule napi from a workqueue. Afterwards, softirqs are not run
in a deterministic time frame. The problem is the same as what was
described in commit ec13ee80

 ("virtio_net: invoke softirqs after
__napi_schedule") and this patch applies the same fix to i40e.

Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>