xfrm: add state hashtable keyed by seq

When creating new states with seq set in xfrm_usersa_info, we walk
through all the states already installed in that netns to find a
matching ACQUIRE state (__xfrm_find_acq_byseq, called from
xfrm_state_add). This causes severe slowdowns on systems with a large
number of states.

This patch introduces a hashtable using x->km.seq as key, so that the
corresponding state can be found in a reasonable time.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>