crypto: chtls - simplify a bit 'create_flowc_wr_skb()'

Use '__skb_put_data()' instead of rewritting it.
This improves readability.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>