RDMA/mlx5: Introduce mlx5r_cache_rb_key

Switch from using the mkey order to using the new struct as the key to the
RB tree of cache entries.

The key is all the mkey properties that UMR operations can't modify.
Using this key to define the cache entries and to search and create cache
mkeys.

Link: https://lore.kernel.org/r/20230125222807.6921-5-michaelgur@nvidia.com
Signed-off-by: Michael Guralnik <michaelgur@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>