RDMA: Remove minor pahole differences between 32/64

To help automatic detection we want pahole to report the same struct
layouts for 32 and 64 bit compiles. These cases are all implicit
padding added at the end of embedded structs as part of a union.

The added reserved fields have no impact on the ABI.

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