RDMA/mlx5: Separate restrack callbacks initialization from main.c

The restrack code has separate .c, so move callbacks initialization to
that file to improve code locality.

Link: https://lore.kernel.org/r/20200702081809.423482-3-leon@kernel.org


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