RDMA/mlx4: Fix uABI structure layouts for 32/64 compat

rss_caps in struct mlx4_uverbs_ex_query_device_resp is misaligned on
32 bit compared to 64 bit, add explicit padding.

The rss caps were introduced recently and are very rarely used in user
space, mainly for DPDK.

We don't expect there to be a real 32 bit user, so this change is done
without compat considerations.

Fixes: 09d208b2

 ("IB/mlx4: Add report for RSS capabilities by vendor channel")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>