net: skbuff: hide nf_trace and ipvs_property

Accesses to nf_trace and ipvs_property are already wrapped
by ifdefs where necessary. Don't allocate the bits for those
fields at all if possible.

Acked-by: Florian Westphal <fw@strlen.de>
Acked-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>