IB/mlx5: Add mmap support for VAR

Add mmap support for VAR, it uses the 'offset' command mode with
involvement of IB core APIs to find the previously allocated mmap entry.

Link: https://lore.kernel.org/r/20191212110928.334995-6-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>