IB/mlx4: Create slave AH's directly

Since slave GID's do not exist in the core gid table we can no longer use
the core code to help do this without creating inconsistencies. Directly
create the AH using mlx4 internal APIs.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>