src: Make flush command selective of the set structure type

src: Make flush command selective of the set structure type

The internal set infrastructure is used for sets, maps and flow tables.
The flush command requires the set type but currently it works for all
of them. E.g. if there is a set named 's' in a table 't' the following
command shouldn't be valid but still executes:

$ nft flush flow table t s

This patch makes the flush command selective so 'flush flow table' only
works in flow tables and so on.

Fixes: 6d37dae ("parser_bison: Allow flushing maps")
Fixes: 2daa0ee ("parser_bison: Allow flushing flow tables")
Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>