[IPSEC]: Fix zero return value in xfrm_lookup on error

Further testing shows that my ICMP relookup patch can cause xfrm_lookup
to return zero on error which isn't very nice since it leads to the caller
dying on null pointer dereference.  The bug is due to not setting err
to ENOENT just before we leave xfrm_lookup in case of no policy.

This patch moves the err setting to where it should be.

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