RDMA/restrack: Make is_visible_in_pid_ns() as an API

Remove is_visible_in_pid_ns() from nldev.c and make it as a restrack API,
so that it can be taken advantage by other parts like counter.

Signed-off-by: Mark Zhang <markz@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>