src: fix reset element support for interval set type

src: fix reset element support for interval set type

Running reset command yields on an interval (rbtree) set yields:
nft reset element inet filter rbtreeset {1.2.3.4}
BUG: unhandled op 8

This is easy to fix, CMD_RESET doesn't add or remove so it should be
treated like CMD_GET.

Unfortunately, this still doesn't work properly:

nft get element inet filter rbset {1.2.3.4}
returns:
 ... elements = { 1.2.3.4 }

but its expected that "get" and "reset" also return stateful objects
associated with the element.  This works for other set types, but for
rbtree, the list of statements gets lost during segtree processing.

After fix, get/reset returns:
  elements = { 1.2.3.4 counter packets 10 ...

A follow up patch will add a test case.

Fixes: 83e0f4402fb7 ("Implement 'reset {set,map,element}' commands")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>