Rename rxe_mc_grp to rxe_mcg. Rename rxe_mc_elem to rxe_mca.
These can be read 'multicast group' and 'multicast attachment'.
'elem' collided with the use of elem in rxe pools and was a little
confusing.

Link: https://lore.kernel.org/r/20220127213755.31697-4-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>