net: snmp: fix 64bit stats on 32bit arches

I accidentally replaced BH disabling by preemption disabling
in SNMP_ADD_STATS64() and SNMP_UPD_PO_STATS64() on 32bit builds.

For 64bit stats on 32bit arch, we really need to disable BH,
since the "struct u64_stats_sync syncp" might be manipulated
both from process and BH contexts.

Fixes: 6aef70a8 ("net: snmp: kill various STATS_USER() helpers")
Reported-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>