xfrm: policy: add sequence count to sync with hash resize

Once xfrm_policy_lookup_bytype doesn't grab xfrm_policy_lock anymore its
possible for a hash resize to occur in parallel.

Use sequence counter to block lookup in case a resize is in
progress and to also re-lookup in case hash table was altered
in the mean time (might cause use to not find the best-match).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>