RDMA/rxe: Cleanup rxe_mcast.c

Finish adding subroutine comment headers to subroutines in
rxe_mcast.c. Make minor api change cleanups.

Link: https://lore.kernel.org/r/20220223230706.50332-5-rpearsonhpe@gmail.com


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