hv_netvsc: Add check for kvmalloc_array

As the potential failure of the kvmalloc_array(),
it should be better to check and restore the 'data'
if fails in order to avoid the dereference of the
NULL pointer.

Fixes: 6ae7467 ("hv_netvsc: Add per-cpu ethtool stats for netvsc")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Link: https://lore.kernel.org/r/20220314020125.2365084-1-jiasheng@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/hyperv/netvsc_drv.c

@@ -1587,6 +1587,9 @@ static void netvsc_get_ethtool_stats(struct net_device *dev,
 	pcpu_sum = kvmalloc_array(num_possible_cpus(),
 				  sizeof(struct netvsc_ethtool_pcpu_stats),
 				  GFP_KERNEL);
+	if (!pcpu_sum)
+		return;
+
 	netvsc_get_pcpu_stats(dev, pcpu_sum);
 	for_each_present_cpu(cpu) {
 		struct netvsc_ethtool_pcpu_stats *this_sum = &pcpu_sum[cpu];