ixgbe: Minor refactor of RSC

This change addresses several issue.

First I had left the use of the next and prev skb pointers floating around
in the code and they were overdue to be pulled since I had rewritten the
RSC code in the out-of-tree driver some time ago to address issues brought
up by David Miller in regards to this.

I am also now defaulting to always leaving the first buffer unmapped on any
packet and then unmapping it after we read the EOP descriptor.  This allows
a simplification of the path with less branching.

Instead of counting packets received the code was changed some time ago to
track the number of buffers received.  This leads to inaccurate counting
when you compare numbers of packets received by the hardware versus what is
tracked by the software.  To correct this I am revising things so that the
append_cnt value for RSC accurately tracks the number of frames received.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>