RDMA/restrack: Remove PID namespace support

IB resources are bounded to IB device and file descriptors, both entities
are unaware to PID namespaces and to task lifetime.

The difference in model caused to unpredictable behavior for the following
scenario:
 1. Create FD and context
 2. Share it with ephemeral child
 3. Create any object and exit that child

The end result of this flow, that those newly created objects will be
tracked by restrack, but won't be visible for users because task_struct
associated with them already exited.

The right thing is to rely on net namespace only for any filtering
purposes and drop PID namespace.

Link: https://lore.kernel.org/r/20191010071105.25538-2-leon@kernel.org


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