RDMA/cma: Fix use after free race in roce multicast join

The roce path triggers a work queue that continues to touch the id_priv
but doesn't hold any reference on it. Futher, unlike in the IB case, the
work queue is not fenced during rdma_destroy_id().

This can trigger a use after free if a destroy is triggered in the
incredibly narrow window after the queue_work and the work starting and
obtaining the handler_mutex.

The only purpose of this work queue is to run the ULP event callback from
the standard context, so switch the design to use the existing
cma_work_handler() scheme. This simplifies quite a lot of the flow:

- Use the cma_work_handler() callback to launch the work for roce. This
  requires generating the event synchronously inside the
  rdma_join_multicast(), which in turn means the dummy struct
  ib_sa_multicast can become a simple stack variable.

- cm_work_handler() used the id_priv kref, so we can entirely eliminate
  the kref inside struct cma_multicast. Since the cma_multicast never
  leaks into an unprotected work queue the kfree can be done at the same
  time as for IB.

- Eliminating the general multicast.ib requires using cma_set_mgid() in a
  few places to recompute the mgid.

Fixes: 3c86aa70 ("RDMA/cm: Add RDMA CM support for IBoE devices")
Link: https://lore.kernel.org/r/20200902081122.745412-9-leon@kernel.orgSigned-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>