netns: add support of NETNSA_TARGET_NSID

Like it was done for link and address, add the ability to perform get/dump
in another netns by specifying a target nsid attribute.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>