RDMA/cm: Fix checking for allowed duplicate listens

The test here typod the cm_id_priv to use, it used the one that was
freshly allocated. By definition the allocated one has the matching
cm_handler and zero context, so the condition was always true.

Instead check that the existing listening ID is compatible with the
proposed handler so that it can be shared, as was originally intended.

Fixes: 067b171b ("IB/cm: Share listening CM IDs")
Link: https://lore.kernel.org/r/20200310092545.251365-3-leon@kernel.org


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