Fix device memory flows so that only once there will be no live mmaped
VA to a given allocation the matching object will be destroyed.

This prevents a potential scenario that existing VA that was mmaped by
one process might still be used post its deallocation despite that it's
owned now by other process.

The above is achieved by integrating with IB core APIs to manage
mmap/munmap. Only once the refcount will become 0 the DM object and its
underlay area will be freed.

Fixes: 3b113a1e ("IB/mlx5: Support device memory type attribute")
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Link: https://lore.kernel.org/r/20191212100237.330654-3-leon@kernel.orgSigned-off-by: Doug Ledford <dledford@redhat.com>