xfrm: Use __xfrm_policy_link in xfrm_policy_insert

For a long time we couldn't actually use __xfrm_policy_link in
xfrm_policy_insert because the latter wanted to do hashing at
a specific position.

Now that __xfrm_policy_link no longer does hashing it can now
be safely used in xfrm_policy_insert to kill some duplicate code,
finally reuniting general policies with socket policies.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>