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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>