[IPSEC]: Fix potential dst leak in xfrm_lookup

If we get an error during the actual policy lookup we don't free the
original dst while the caller expects us to always free the original
dst in case of error.

This patch fixes that.

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