RDMA/restrack: Rewrite PID namespace check to be reliable

RDMA/restrack: Rewrite PID namespace check to be reliable

task_active_pid_ns() is wrong API to check PID namespace because it
posses some restrictions and return PID namespace where the process
was allocated. It created mismatches with current namespace, which
can be different.

Rewrite whole rdma_is_visible_in_pid_ns() logic to provide reliable
results without any relation to allocated PID namespace.

Fixes: 8be565e65fa9 ("RDMA/nldev: Factor out the PID namespace check")
Fixes: 6a6c306a09b5 ("RDMA/restrack: Make is_visible_in_pid_ns() as an API")
Reviewed-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Link: https://lore.kernel.org/r/20190815083834.9245-4-leon@kernel.org
Signed-off-by: Doug Ledford <dledford@redhat.com>