IB/mlx5: Introduce VAR object and its alloc/destroy methods

Introduce VAR object and its alloc/destroy KABI methods. The internal
implementation uses the IB core API to manage mmap/munamp calls.

Link: https://lore.kernel.org/r/20191212110928.334995-5-leon@kernel.org


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