Let's also fix IPsec PMTU storage.  When we get an MTU update for an
xfrm_dst, it should be done to the top dst, not the bottom dst.

For example, when we get a need-to-frag message for host C behind
a our IPsec peer B, we should be updating the dst entry for C and
not B as we do now.

I've removed the boundary checks since the same checks are done
in ipv[46]/route.c already.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Cheers,
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