IB/core: Cut down single member ib_cache structure

Given that ib_cache structure has only single member now, merge the cache
lock directly in the ib_device.

Link: https://lore.kernel.org/r/20191212113024.336702-4-leon@kernel.org


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