[NETFILTER] nfnetlink: use highest bit of nfa_type to indicate nested TLV

As Henrik Nordstrom pointed out, all our efforts with "split endian" (i.e.
host byte order tags, net byte order values) are useless, unless a parser
can determine whether an attribute is nested or not.

This patch steals the highest bit of nfattr.nfa_type to indicate whether
the data payload contains a nested nfattr (1) or not (0).

This will break userspace compatibility, but luckily no kernel with
nfnetlink was released so far.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>