RDMA/core: Get xmit slave for LAG

Add a call to rdma_lag_get_ah_roce_slave() when the address handle is
created. Lower driver can use it to select the QP's affinity port.

Link: https://lore.kernel.org/r/20200430192146.12863-15-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>