RDMA/rxe: Make pool lookup and alloc APIs type safe

RDMA/rxe: Make pool lookup and alloc APIs type safe

The allocate, lookup index, lookup key and cleanup routines in rxe_pool.c
currently are not type safe against relocating the pelem field in the
objects. Planned changes to move allocation of objects into rdma-core make
addressing this a requirement.

Use the elem_offset field in rxe_type_info make these APIs safe against
moving the pelem field.

Link: https://lore.kernel.org/r/20201216231550.27224-5-rpearson@hpe.com
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>