[IPSEC]: Check validity of direction in xfrm_policy_byid

The function xfrm_policy_byid takes a dir argument but finds the policy
using the index instead.  We only use the dir argument to update the
policy count for that direction.  Since the user can supply any value
for dir, this can corrupt our policy count.

I know this is the problem because a few days ago I was deleting
policies by hand using indicies and accidentally typed in the wrong
direction.  It still deleted the policy and at the time I thought
that was cool.  In retrospect it isn't such a good idea :)

I decided against letting it delete the policy anyway just in case
we ever remove the connection between indicies and direction.

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