i40e/i40evf: Refactor the receive routines

Split the receive hot path code into two, one for packet split and one
for single buffer. This improves receive performance since we only need
to check if the ring is in packet split mode once per NAPI poll time,
not several times per packet. The single buffer code is further improved
by the removal of a bunch of code and several variables that are not
needed. On a receive-oriented test this can improve single-threaded
throughput.

Also refactor the packet split receive path to use a fixed buffer for
headers, like ixgbe does. This vastly reduces the number of DMA mappings
and unmappings we need to do, allowing for much better performance in
the presence of an IOMMU.

Lastly, correct packet split descriptor types now that we are actually
using them.

Change-ID: I3a194a93af3d2c31e77ff17644ac7376da6f3e4b
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>