mnl: do not send set size when set is constant set

mnl: do not send set size when set is constant set

When turning element range into the interval representation based on
singleton elements for the rbtree tree set backend, userspace adjusts
the size to the internal kernel implementation.

For constant sets, this is leaking an internal kernel implementation
detail that is fixed by kernel patch ("netfilter: nf_tables: fix set
size with rbtree backend"). For non-constant sets, set size is just
broken.

This patch is required by the follow up patch ("src: rework singleton
interval transformation to reduce memory consumption").

On top of this, constant sets cannot be updated once they are bound, set
size is not useful in this case. Remove this implicit set size for
constant sets.

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