enic: make vnic_wq_buf doubly linked

This patch makes vnic_wq_buf doubly liked list. This is needed for dma_mapping
error check, in case some frag's dma map fails, we need to move back and remove
previously queued buffers.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/ethernet/cisco/enic/vnic_wq.c
Simple merge

diff --cc drivers/net/ethernet/cisco/enic/vnic_wq.h
Simple merge