Fix an overflow bug in vmxnet3 Tx descriptor

This patch fixes a bug where a 16K buffer on a Tx descriptor was overflowing
into the 'gen' bit in the descriptor thereby corrupting the descriptor and
stalling the transmit ring.

Signed-off-by: Bhavesh Davda <bhavesh@vmware.com>
Signed-off-by: Shreyas Bhatewara <sbhatewara@vmware.com>
Signed-off-by: Matthew Delco <delcoM@vmware.com>
Signed-off-by: Ronghua Zhang <ronghua@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>