RDMA/rxe: Replace obj by elem in declaration

Fix a harmless typo replacing obj by elem in the cleanup fields.  This has
no effect but is confusing.

Link: https://lore.kernel.org/r/20220304000808.225811-6-rpearsonhpe@gmail.com


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