[IPSEC]: Revert ip{,6}_append_data() dst_mtu() conversion.

[IPSEC]: Revert ip{,6}_append_data() dst_mtu() conversion.

This patch is wrong.  This is the *one* place where we do need to
use the path MTU.  The reason is that when the packet is fragmented
we only pay for the IPsec overhead once over all and not once for
each fragment.

Please revert it for now.

The trailer_len in ip_append_data is not quite right as the trailer's
length depends on the length of the entire packet.  However, it should
be harmless since ESP knows how to extend the packet when necessary.

Cset exclude: herbert@gondor.apana.org.au|ChangeSet|20050315181807|13007