RDMA/rxe: Replace grp by mcg, mce by mca

Replace 'grp' by 'mcg', 'mce' by 'mca'.  Shorten subroutine names in
rxe_mcast.c.  These name uses are more in line with other object names
used.

Link: https://lore.kernel.org/r/20220208211644.123457-4-rpearsonhpe@gmail.com


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