RDMA/cma: Consolidate the destruction of a cma_multicast in one place

Two places were open coding this sequence, and also pull in
cma_leave_roce_mc_group() which was called only once.

Link: https://lore.kernel.org/r/20200902081122.745412-8-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>