hv_netvsc: Don't assume cpu_possible_mask is dense

Current code allocates the pcpu_sum array with size num_possible_cpus().
This code assumes the cpu_possible_mask is dense, which is not true in
the general case per [1]. If cpu_possible_mask is sparse, the array
might be indexed by a value beyond the size of the array.

However, the configurations that Hyper-V provides to guest VMs on x86
and ARM64 hardware, in combination with how architecture specific code
assigns Linux CPU numbers, *does* always produce a dense cpu_possible_mask.
So the dense assumption is not currently causing failures. But for
robustness against future changes in how cpu_possible_mask is populated,
update the code to no longer assume dense.

The correct approach is to allocate and initialize the array using size
"nr_cpu_ids". While this leaves unused array entries corresponding to
holes in cpu_possible_mask, the holes are assumed to be minimal and hence
the amount of memory wasted by unused entries is minimal.

[1] https://lore.kernel.org/lkml/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20241003035333.49261-6-mhklinux@outlook.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/hyperv/netvsc_drv.c

@@ -1557,7 +1557,7 @@ static void netvsc_get_ethtool_stats(struct net_device *dev,
 		data[i++] = xdp_tx;
 	}
 
-	pcpu_sum = kvmalloc_array(num_possible_cpus(),
+	pcpu_sum = kvmalloc_array(nr_cpu_ids,
 				  sizeof(struct netvsc_ethtool_pcpu_stats),
 				  GFP_KERNEL);
 	if (!pcpu_sum)