src: Allow passing the parent set to set_expr_alloc()

src: Allow passing the parent set to set_expr_alloc()

Usually one wants to at least initialize set_flags from the parent, so
make allocation of a set's set expression more convenient.

The idea to do this came when fixing an issue with output formatting of
larger anonymous sets in nft monitor: Since
netlink_events_cache_addset() didn't initialize set_flags,
calculate_delim() didn't detect it's an anonymous set and therefore
added newlines to the output.

Reported-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Fixes: a9dc3ceabc10f ("expression: print sets and maps in pretty format")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Arturo Borrero Gonzalez <arturo@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>