netfilter: nft_set_rbtree: handle element re-addition after deletion

commit d2df92e98a34a5619dadd29c6291113c009181e7 upstream.

The existing code selects no next branch to be inspected when
re-inserting an inactive element into the rb-tree, looping endlessly.
This patch restricts the check for active elements to the EEXIST case
only.

Fixes: e701001e ("netfilter: nft_rbtree: allow adjacent intervals with dynamic updates")
Reported-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Tested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>