ixgbevf: fold ixgbevf_pull_tail into ixgbevf_add_rx_frag

This change folds the ixgbevf_pull_tail call into ixgbevf_add_rx_frag.  The
advantage to doing this is that the fragment doesn't have to be modified
after it is added to the skb.

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