RDMA/ucma: Add missing locking around rdma_leave_multicast()

All entry points to the rdma_cm from a ULP must be single threaded,
even this error unwinds. Add the missing locking.

Fixes: 7c119107 ("RDMA/ucma: Put a lock around every call to the rdma_cm layer")
Link: https://lore.kernel.org/r/20200818120526.702120-11-leon@kernel.org


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