ixgbe: always write DMA for single_mapped value with skb

This change makes it so that we always write the DMA address for the skb
itself on the same tx_buffer struct that the skb is written on.  This way
we don't need the MAPPED_AS_PAGE flag and we always know it will be the
first DMA value that we will have to unmap.

In addition I have found an issue in which we were leaking a DMA mapping if
the value happened to be 0 which is possible on some platforms.  In order
to resolve that I have updated the transmit path to use the length instead
of the DMA mapping in order to determine if a mapping is actually present.

One other tweak in this patch is that it only writes the olinfo information
on the first descriptor.  As it turns out it isn't necessary to write it
for anything but the first descriptor so there is no need to carry it
forward.

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>