RDMA/mlx5: Refactor MR descriptors allocation

Improve code readability using static helpers for each memory region
type. Re-use the common logic to get smaller functions that are easy
to maintain and reduce code duplication.
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>