netfilter: nfnetlink: New NFNLA_HOOK_INFO_DESC helper

Introduce a helper routine adding the nested attribute for use by a
second caller later.

Note how this introduces cancelling of 'nest2' for categorical reasons.
Since always followed by cancelling of the outer 'nest', it is
technically not needed.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>