RDMA/rxe: Add elem_offset field to rxe_type_info

The rxe verbs objects each include an rdma-core object 'ib_xxx'
and a rxe_pool_entry 'pelem' in addition to rxe specific data.
Originally these all had pelem first and ib_xxx second. Currently
about half have ib_xxx first and half have pelem first. Saving
the offset of the pelem field in rxe_type info will enable making
the rxe_pool APIs type safe as the pelem field continues to vary.

Link: https://lore.kernel.org/r/20201216231550.27224-4-rpearson@hpe.com


Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>