ixgbe: Make better use of memory allocations in one-buffer mode w/ RSC

This patch improves the memory utilization with RSC when in one-buffer
mode.  This is accomplished by making the default buffer sizes match up
with the standard memory allocation sizes minus 1K for shared info and
padding overhead.  By doing this CPU utilization when doing large receives
can be reduced by as much as 8%.

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