ixgbevf: Combine the logic for post Rx processing into single function

This patch cleans up ixgbevf_clean_rx_irq() by merging several similar
operations into a new function - ixgbevf_process_skb_fields().

CC: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>