RDMA/mlx4: Delete duplicated offsetofend implementation

Convert mlx4 to use in-kernel offsetofend() instead
of its duplicated implementation.

Link: https://lore.kernel.org/r/20200310091438.248429-3-leon@kernel.org


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