RDMA/mlx5: Use offsetofend() instead of duplicated variant

Convert mlx5 driver to use offsetofend() instead of its duplicated
variant.

Link: https://lore.kernel.org/r/20200310091438.248429-5-leon@kernel.org


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