xfrm: revert ipv4 mtu determination to dst_mtu

xfrm: revert ipv4 mtu determination to dst_mtu

In commit 9155960d636b6dbc6594c4a425e21256898cfa41 ("xfrm: introduce
helper for safe determination of mtu") I switched the determination of
ipv4 mtus from dst_mtu to ip_skb_dst_mtu. This was an error because in
case of IP_PMTUDISC_PROBE we fall back to the interface mtu, which is
never correct for ipv4 ipsec.

This patch partly reverts 9155960d636b6dbc6594c4a425e21256898cfa41
("xfrm: introduce helper for safe determination of mtu").

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>