RDMA/rxe: Use structs to describe the uABI instead of opencoding

Open coding pointer math is not acceptable for describing the uABI in
RDMA. Provide structs for all the cases.

The udata is casted to the struct as close to the verbs entry point
as possible for maximum clarity. Function signatures and so forth
are revised to allow for this.

Tested-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>