i40e: Reorder logic for coalescing RS bits

i40e: Reorder logic for coalescing RS bits

Orabug: 24568124

This patch reorders the logic at the end of i40e_tx_map to address the
fact that the logic was rather convoluted and much larger than it needed
to be.

In order to try and coalesce the code paths I have updated some of the
comments and repurposed some of the variables in order to reduce
unnecessary overhead.

This patch does the following:
1.  Quit tracking skb->xmit_more with a flag, just max out packet_stride
2.  Drop tail_bump and do_rs and instead just use desc_count and td_cmd
3.  Pull comments from ixgbe that make need for wmb() more explicit.

Change-ID: Ic7da85ec75043c634e87fef958109789bcc6317c
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 1dc8b538795fcf92161cc056ceebb852cf375b17)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	drivers/net/ethernet/intel/i40e/i40e_txrx.c
	drivers/net/ethernet/intel/i40evf/i40e_txrx.c

Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>