netlabel: Add a generic way to create ordered linked lists of network addrs

Create an ordered IP address linked list mechanism similar to the core
kernel's linked list construct.  The idea behind this list functionality
is to create an extensibile linked list ordered by IP address mask to
ease the matching of network addresses.  The linked list is ordered with
larger address masks at the front of the list and shorter address masks
at the end to facilitate overriding network entries with individual host
or subnet entries.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Reviewed-by: James Morris <jmorris@namei.org>