RDMA/rxe: Save object pointer in pool element

In rxe_pool.c currently there are many cases where it is necessary to
compute the offset from a pool element struct to the object containing it
in a type independent way where the offset is different for each type.  By
saving a pointer to the object when they are created extra work can be
saved.

Link: https://lore.kernel.org/r/20211103050241.61293-5-rpearsonhpe@gmail.com


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