netfilter: uapi: correct UNTRACKED conntrack state bit number

nft_ct exposes this bit to userspace.  This used to be

  #define NF_CT_STATE_UNTRACKED_BIT              (1 << (IP_CT_NUMBER + 1))
  (IP_CT_NUMBER is 5, so this was 0x40)

.. but this got changed to 8 (0x100) when the untracked object got removed.
Replace this with a literal 6 to prevent further incompatible changes
in case IP_CT_NUMBER ever increases.

Fixes: cc41c84b

 ("netfilter: kill the fake untracked conntrack objects")
Reported-by: Li Shuang <shuali@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>