[IPv6] IPsec: fix pmtu calculation of esp

It is a simple bug which uses the wrong member.

This bug does not seriously affect ordinary use of IPsec.
But it is important to pass IPv6 ready logo phase-2
conformance test of IPsec SGW.

Signed-off-by: Kazunori MIYAZAWA <miyazawa@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>