[IPCOMP]: Fix truesize after decompression

The truesize check has uncovered the fact that we forgot to update truesize
after pskb_expand_head.  Unfortunately pskb_expand_head can't update it for
us because it's used in all sorts of different contexts, some of which would
not allow truesize to be updated by itself.

So the solution for now is to simply update it in IPComp.

This patch also changes skb_put to __skb_put since we've just expanded
tailroom by exactly that amount so we know it's there (but gcc does not).

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