vmxnet3: fix tx data ring copy for variable size

vmxnet3: fix tx data ring copy for variable size

Orabug: 25639933

'Commit 3c8b3efc061a ("vmxnet3: allow variable length transmit data ring
buffer")' changed the size of the buffers in the tx data ring from a
fixed size of 128 bytes to a variable size.

However, while copying data to the data ring, vmxnet3_copy_hdr continues
to carry the old code that assumes fixed buffer size of 128. This patch
fixes it by adding correct offset based on the actual data ring buffer
size.

Signed-off-by: Guolin Yang <gyang@vmware.com>
Signed-off-by: Shrikrishna Khare <skhare@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit ff2e7d5d51469e98196f7933c83b781e96517e7c)
Signed-off-by: Brian Maly <brian.maly@oracle.com>