zebra, lib: new API to get absolute netns val from relative netns val

zebra, lib: new API to get absolute netns val from relative netns val

when receiving a netlink API for an interface in a namespace, this
interface may come with LINK_NSID value, which means that the interface
has its link in an other namespace. Unfortunately, the link_nsid value
is self to that namespace, and there is a need to know what is its
associated nsid value from the default namespace point of view.
The information collected previously on each namespace, can then be
compared with that value to check if the link belongs to the default
namespace or not.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>