vmxnet3: support multi-segment transmit

vmxnet3: support multi-segment transmit

Change sending loop to support multi-segment mbufs.
The VMXNET3 api has start-of-packet and end-packet flags, so it
is not hard to send multi-segment mbuf's.

Also, update descriptor in 32 bit value rather than toggling
bitfields which is slower and error prone.
Based on code in earlier driver, and the Linux kernel driver.

Add a compiler barrier to make sure that update of earlier descriptor
are completed prior to update of generation bit on start of packet.

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