netfilter: nft_dynset: relax superfluous check on set updates

[ Upstream commit 7b1394892de8d95748d05e3ee41e85edb4abbfa1 ]

Relax this condition to make add and update commands idempotent for sets
with no timeout. The eval function already checks if the set element
timeout is available and updates it if the update command is used.

Fixes: 22fe54d5 ("netfilter: nf_tables: add support for dynamic set updates")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>