netfilter: nf_tables: always use an upper set size for dynsets

nft rejects rules that lack a timeout and a size limit when they're used
to add elements from packet path.

Pick a sane upperlimit instead of rejecting outright.
The upperlimit is visible to userspace, just as if it would have been
given during set declaration.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>