sset: Make sset iteration check the type of the nodes it's iterating.

sset: Make sset iteration check the type of the nodes it's iterating.

Without this additional check, SSET_FOR_EACH is happy to iterate over any
type that has a struct hmap member named 'map'.  We have a number of these:
hmapx, shash, simap, and smap, in addition to sset.

I guess another approach would be give each of these a unique member name,
but the short name 'map' is nice too.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>