vmxnet3: allow variable length transmit data ring buffer

vmxnet3: allow variable length transmit data ring buffer

Orabug: 25639933

vmxnet3 driver supports transmit data ring viz. a set of fixed size
buffers used by the driver to copy packet headers. Small packets that
fit these buffers are copied into these buffers entirely.

Currently this buffer size of fixed at 128 bytes. This patch extends
transmit data ring implementation to allow variable length transmit
data ring buffers. The length of the buffer is read from the emulation
during initialization.

Signed-off-by: Sriram Rangarajan <rangarajans@vmware.com>
Signed-off-by: Shrikrishna Khare <skhare@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 3c8b3efc061a745d888869dc3462ac4f7dd582d9)
Signed-off-by: Brian Maly <brian.maly@oracle.com>