RDMA/mlx5: Reorganize mlx5_ib_reg_user_mr()

This function handles an ODP and regular MR flow all mushed together, even
though the two flows are quite different. Split them into two dedicated
functions.

Link: https://lore.kernel.org/r/20201130075839.278575-5-leon@kernel.org


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