mnl: add mnl_nft_setelem_batch_flush() and use it from netlink_flush_setelems()

mnl: add mnl_nft_setelem_batch_flush() and use it from netlink_flush_setelems()

Commit 8bd99f2fca7e ("mnl: don't send empty set elements netlink message
to kernel") broke set flush because we still need to send the netlink
message with no elements to flush sets.

To avoid more whack-a-mole games, add a new explicit function
mnl_nft_setelem_batch_flush() that is used to request a set flush,
instead of reusing the one that allows us to explicitly delete given set
elements.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>