ipfrag: really prevent allocation on netns exit

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>