RDMA/cxgb4: Add support for active and passive open connection with IPv6 address

Add new cpl messages, cpl_act_open_req6 and cpl_t5_act_open_req6, for
initiating active open connections.

Use LLD api cxgb4_create_server and cxgb4_create_server6 for
initiating passive open connections. Similarly use cxgb4_remove_server
to remove the passive open connections in place of listen_stop.

Add support for iWARP over VLAN device and enable IPv6 support on VLAN device.

Make use of import_ep in c4iw_reconnect.

Signed-off-by: Vipul Pandya <vipul@chelsio.com>

[ Fix build when IPv6 is disabled and make sure iw_cxgb4 is not built-in
  when ipv6 is a module.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>