RDMA/core: Add LAG functionality

Add support to get the RoCE LAG xmit slave by building skb of the RoCE
packet and call to master_get_xmit_slave.  If driver wants to get the
slave assume all slaves are available, then need to set
RDMA_LAG_FLAGS_HASH_ALL_SLAVES in flags.

Link: https://lore.kernel.org/r/20200430192146.12863-14-maorg@mellanox.com
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>