cxgbe/tom: Miscellaneous updates for TOE+IPv6 support (more to follow).
- Teach find_best_mtu_idx() to deal with IPv6 endpoints.
- Install correct protosw in offloaded TCP/IPv6 sockets when DDP is enabled.
- Move set_tcp_ddp_ulp_mode to t4_tom.c so that t4_tom.h can be included without having to drag in t4_msg.h too. This was bothering the iWARP driver for some reason.
MFC after: 1 week