netfilter: nftables: relax check for stateful expressions in set definition

Restore the original behaviour where users are allowed to add an element
with any stateful expression if the set definition specifies no stateful
expressions. Make sure upper maximum number of stateful expressions of
NFT_SET_EXPR_MAX is not reached.

Fixes: 8cfd9b0f ("netfilter: nftables: generalize set expressions support")
Fixes: 48b0ae04 ("netfilter: nftables: netlink support for several set element expressions")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>