vmxnet3: restore Tx data ring support

vmxnet3: restore Tx data ring support

Tx data ring support was removed in a previous change that
added multi-seg transmit.  This change adds it back.

According to the original commit (2e849373), 64B pkt
rate with l2fwd improved by ~20% on an Ivy Bridge
server at which point we start to hit some bottleneck
on the rx side.

I also re-did the same test on a different setup (Haswell
processor, ~2.3GHz clock rate) on top of the master
and still observed ~17% performance gains.

Fixes: 7ba5de417e3c ("vmxnet3: support multi-segment transmit")

Signed-off-by: Yong Wang <yongwang@vmware.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>