[IPSEC]: Fix length check in xfrm_parse_spi

Currently xfrm_parse_spi requires there to be 16 bytes for AH and ESP.
In contrived cases there may not actually be 16 bytes there since the
respective header sizes are less than that (8 and 12 currently).

This patch changes the test to use the actual header length instead of 16.

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