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