netfilter: nft_exthdr: Fix non-linear header modification

netfilter: nft_exthdr: Fix non-linear header modification

Fix skb_ensure_writable() size. Don't use nft_tcp_header_pointer() to
make it explicit that pointers point to the packet (not local buffer).

Fixes: 99d1712bc41c ("netfilter: exthdr: tcp option set support")
Fixes: 7890cbea66e7 ("netfilter: exthdr: add support for tcp option removal")
Cc: stable@vger.kernel.org
Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>