ixgbevf: Make next_to_watch a pointer and adjust memory barriers to avoid races

This change is meant to address several race issues that become possible
because next_to_watch could possibly be set to a value that shows that the
descriptor is done when it is not.  In order to correct that we instead make
next_to_watch a pointer that is set to NULL during cleanup, and set to the
eop_desc after the descriptor rings have been written.

To enforce proper ordering the next_to_watch pointer is not set until after
a wmb writing the values to the last descriptor in a transmit.  In order to
guarantee that the descriptor is not read until after the eop_desc we use the
read_barrier_depends which is only really necessary on the alpha architecture.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>