netfilter: nft_set_rbtree: check for inactive element after flag mismatch

Otherwise, we hit bogus ENOENT when removing elements.

Fixes: e701001e ("netfilter: nft_rbtree: allow adjacent intervals with dynamic updates")
Reported-by: Václav Zindulka <vaclav.zindulka@tlapnet.cz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>