zebra: upon NS creation, collect the NSID via netlink

zebra: upon NS creation, collect the NSID via netlink

A NS identifier is collected by netlink. This identifier is a 32 bit
identifier that is either generated by the kernel (if not set) or
manually set by a set netlink command. The commit here is getting the
NSID from the newly created NS. If the linux option to create or get a
new NSID from the kernel does not exist, then the NSID is locally
genrated.

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