ixgbevf: make the first tx_buffer a repository for most of the skb info

This change makes it so that the first tx_buffer structure acts as a
central storage location for most of the info about the skb we are about
to transmit.

In addition this patch makes tx_flags part of the ixgbevf_tx_buffer struct.
This allows us to use the flags directly from the stucture and as result
removes the tx_flags parameter from some functions. Also as a cleanup
mapped_as_page is folded into tx_flags and some unused flags were removed.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>