netfilter: flowtable: consolidate skb_try_make_writable() call

Fetch the layer 4 header size to be mangled by NAT when building the
tuple, then use it to make writable the network and the transport
headers. After this update, the NAT routines now assumes that the skbuff
area is writable. Do the pointer refetch only after the single
skb_try_make_writable() call.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>