RDMA/rxe: Replace pool key by rxe->mcg_tree

Continuing to decouple mcg from rxe pools. Create red-black tree code in
rxe_mcast.c to hold mcg index. Replace pool key calls by calls to local
red-black routines.

Link: https://lore.kernel.org/r/20220208211644.123457-6-rpearsonhpe@gmail.com


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