[IPSEC]: Policy list disorder

The recent hashing introduced an off-by-one bug in policy list insertion.
Instead of adding after the last entry with a lesser or equal priority,
we're adding after the successor of that entry.

This patch fixes this and also adds a warning if we detect a duplicate
entry in the policy list.  This should never happen due to this if clause.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>