RDMA/ucma: Remove mc_list and rely on xarray

It is not really necessary to keep a linked list of mcs associated with
each context when we can just scan the xarray to find the right things.

The removes another overloading of file->mut by relying on the xarray
locking for mc instead.

Link: https://lore.kernel.org/r/20200818120526.702120-6-leon@kernel.org


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