The PID information can disappear asynchronously because the task can be
killed and moved to zombie state. In this case, PID will be zero in
similar way to the kernel tasks. Recognize such situation where we are
asking to return orphaned object and simply skip filling PID attribute.

As part of this change, document the same scenario in counter.c code.

Link: https://lore.kernel.org/r/20191010071105.25538-3-leon@kernel.orgSigned-off-by: NLeon Romanovsky <leonro@mellanox.com>
Reviewed-by: NJason Gunthorpe <jgg@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>