Collect cleanup code for struct rxe_mca into a subroutine,
__rxe_cleanup_mca() called in rxe_detach_mcg() in rxe_mcast.c.

Link: https://lore.kernel.org/r/20220223230706.50332-4-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>