ipcomp: Remove spurious truesize increase

When I made ipcomp use frags, I forgot to take out the original
truesize update that was added for pskb_expand_head.  As we no
longer expand the head of skb, that update should have been removed.

This bug is not related to the truesize warnings since we only
made it bigger than what it should've been.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>