IB/mlx5: Add MR to radix tree in reg_mr_callback

For memory regions that are allocated using reg_umr, the suffix of
mlx5_core_create_mkey isn't being called.  Instead the creation is
completed in a callback function (reg_mr_callback).  This means that
these MRs aren't being added to the MR radix tree.  Add them in the
callback.

Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>