netfilter: xt_hashlimit: use s->file instead of s->private

After switching to the new procfs API, it is supposed to
retrieve the private pointer from PDE_DATA(file_inode(s->file)),
s->private is no longer referred.

Fixes: 1cd67182 ("netfilter/x_tables: switch to proc_create_seq_private")
Reported-by: Sami Farin <hvtaifwkbgefbaei@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Tested-by: Sami Farin <hvtaifwkbgefbaei@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>