[TCP]: MTUprobe: prepare skb fields earlier

They better be valid when call to write_queue functions is made
once things that follow are going in.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>