netfilter: nftables: add set expression flags

netfilter: nftables: add set expression flags

The set flag NFT_SET_EXPR provides a hint to the kernel that userspace
supports for multiple expressions per set element. In the same
direction, NFT_DYNSET_F_EXPR specifies that dynset expression defines
multiple expressions per set element.

This allows new userspace software with old kernels to bail out with
EOPNOTSUPP. This update is similar to ef516e8625dd ("netfilter:
nf_tables: reintroduce the NFT_SET_CONCAT flag"). The NFT_SET_EXPR flag
needs to be set on when the NFTA_SET_EXPRESSIONS attribute is specified.
The NFT_SET_EXPR flag is not set on with NFTA_SET_EXPR to retain
backward compatibility in old userspace binaries.

Fixes: 48b0ae046ee9 ("netfilter: nftables: netlink support for several set element expressions")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>