IB/core: Depend on IPv6 stack to resolve link local address for RoCEv2

IB/core: Depend on IPv6 stack to resolve link local address for RoCEv2

RoCEv1 does not use the IPv6 stack to resolve the link local DGID since it
uses GID address. It forms the DMAC directly from the DGID.

The code became confused and also tried to use this bypass for RoCEv2
packets, however RoCEv2 always uses a IP address in the GID and must
always use ARP or neighbor discovery to get the DMAC address.

Now that rdma_addr_find_l2_eth_by_grh() supports resolving link local
address to find destination mac address, lets make use of it.
This aligns it to how the rest of the IPv6 stack resolves link local
destination IPv6 address.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>