This patchset enables to hash more policies than just non-prefixed
ones: hash policies whose prefix lengths are greater or equal to
configurable thresholds.

These thresholds are configured via netlink message
XFRM_MSG_NEWSPDINFO, attributes XFRMA_SPD_IPV4_HTHRESH and
XFRMA_SPD_IPV6_HTHRESH.

The related iproute2 patch for configuring the thresholds is available
on demand.

Best Regards,
Christophe
----
v2:
- change configuration API from proc to netlink
v3:
- initialize xfrm_policy_hthresh lock
- remove "rebuilding SPD hash table" log
- replace deprecated hlist_add_after by hlist_add_behind
- remove netlink reply to XFRM_MSG_NEWSPDINFO request
v4:
- remove unused variables in xfrm_set_spdinfo
---
 include/net/netns/xfrm.h  |  14 +++++++
 include/net/xfrm.h        |   1 +
 include/uapi/linux/xfrm.h |   7 ++++
 net/xfrm/xfrm_hash.h      |  76 +++++++++++++++++++++++++++++++----- 
 net/xfrm/xfrm_policy.c    | 140 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
 net/xfrm/xfrm_user.c      |  80 ++++++++++++++++++++++++++++++++++++--
 6 files changed, 299 insertions(+), 19 deletions(-)