netfilter: nft_counter: fix reset of counters on 32bit archs

netfilter: nft_counter: fix reset of counters on 32bit archs

nft_counter_reset() calls u64_stats_add() with a negative value to reset
the counter. This will work on 64bit archs, hence the negative value
added will wrap as a 64bit value which then can wrap the stat counter as
well.

On 32bit archs, the added negative value will wrap as a 32bit value and
_not_ wrapping the stat counter properly. In most cases, this would just
lead to a very large 32bit value being added to the stat counter.

Fix by introducing u64_stats_sub().

Fixes: 4a1d3acd6ea8 ("netfilter: nft_counter: Use u64_stats_t for statistic.")
Signed-off-by: Anders Grahn <anders.grahn@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>