RDMA/core: Simplify restrack interface

In the current implementation, we have one restrack root per-device and
all users are simply providing it directly. Let's simplify the interface
and have callers provide the ib_device and internally access the
restrack_root.

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