RDMA/mlx5: Replace open-coded offsetofend() macro

Clean mlx5_ib from open-coded implementations of offsetofend().

Link: https://lore.kernel.org/r/20200730081235.1581127-3-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>