ipfrag: really prevent allocation on netns exit

BugLink: https://bugs.launchpad.net/bugs/1818806

commit f6f2a4a2 upstream.

Setting the low threshold to 0 has no effect on frags allocation,
we need to clear high_thresh instead.

The code was pre-existent to commit 648700f7 ("inet: frags:
use rhashtables for reassembly units"), but before the above,
such assignment had a different role: prevent concurrent eviction
from the worker and the netns cleanup helper.

Fixes: 648700f7 ("inet: frags: use rhashtables for reassembly units")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

net/ipv4/inet_fragment.c

@@ -90,7 +90,7 @@ static void inet_frags_free_cb(void *ptr, void *arg)
 
 void inet_frags_exit_net(struct netns_frags *nf)
 {
-	nf->low_thresh = 0; /* prevent creation of new frags */
+	nf->high_thresh = 0; /* prevent creation of new frags */
 
 	rhashtable_free_and_destroy(&nf->rhashtable, inet_frags_free_cb, NULL);
 }