This adds the fourth and final search class, containing policies
where both saddr and daddr have prefix lengths (i.e., not wildcards).

Inexact policies now end up in one of the following four search classes:

1. "Any:Any" list, containing policies where both saddr and daddr are
   wildcards or have very coarse prefixes, e.g. 10.0.0.0/8 and the like.
2. "saddr:any" list, containing policies with a fixed saddr/prefixlen,
   but without destination restrictions.
   These lists are stored in rbtree nodes; each node contains those
   policies matching saddr/prefixlen.
3. "Any:daddr" list. Similar to 2), except for policies where only the
   destinations are specified.
4. "saddr:daddr" lists, containing only those policies that
   match the given source/destination network.
   The root of the saddr/daddr nodes gets stored in the nodes of the
   'daddr' tree.

This diagram illustrates the list classes, and their
placement in the lookup hierarchy:

    xfrm_pol_inexact_bin = hash(dir,type,family,if_id);
      |
      +---- root_d: sorted by daddr:prefix
      |                 |
      |        xfrm_pol_inexact_node
      |                 |
      |                 +- root: sorted by saddr/prefix
      |                 |              |
      |                 |         xfrm_pol_inexact_node
      |                 |              |
      |                 |              + root: unused
      |                 |              |
      |                 |              + hhead: saddr:daddr policies
      |                 |
      |                 +- coarse policies and all any:daddr policies
      |
      +---- root_s: sorted by saddr:prefix
      |                 |
      |        xfrm_pol_inexact_node
      |                 |
      |                 + root: unused
      |                 |
      |                 + hhead: saddr:any policies
      |
      +---- coarse policies and all any:any policies

lookup for an inexact policy returns pointers to the four relevant list
classes, after which each of the lists needs to be searched for the policy
with the higher priority.

This will only speed up lookups in case we have many policies and a
sizeable portion of these have disjunct saddr/daddr addresses.
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>