[XFRM]: Fix MTU calculation for non-ESP SAs

My IPsec MTU optimization patch introduced a regression in MTU calculation
for non-ESP SAs, the SA's header_len needs to be subtracted from the MTU if
the transform doesn't provide a ->get_mtu() function.

Reported-and-tested-by: Marco Berizzi <pupilla@hotmail.com>

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>