This patch introduces the function xfrm_state_mtu which calculates
the MTU under a specific SA.  This function can be simplified once
we get rid all other uses of get_max_size as we can move the calculation
into the invidual SA type.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>