netfilter: nft_set_rbtree: skip elements in transaction from garbage collection

Skip interference with an ongoing transaction, do not perform garbage
collection on inactive elements. Reset annotated previous end interval
if the expired element is marked as busy (control plane removed the
element right before expiration).

Fixes: 8d8540c4

 ("netfilter: nft_set_rbtree: add timeout support")
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>