RDMA/rxe: Cleanup rxe_pool_entry

Currently three different names are used to describe rxe pool elements.
They are referred to as entries, elems or pelems. This patch chooses one
'elem' and changes the other ones.

Link: https://lore.kernel.org/r/20211103050241.61293-3-rpearsonhpe@gmail.com


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