xdp: Prevent kernel-infoleak in xsk_getsockopt()

xsk_getsockopt() is copying uninitialized stack memory to userspace when
'extra_stats' is 'false'. Fix it. Doing '= {};' is sufficient since currently
'struct xdp_statistics' is defined as follows:

struct xdp_statistics {
__u64 rx_dropped;
__u64 rx_invalid_descs;
__u64 tx_invalid_descs;
__u64 rx_ring_full;
__u64 rx_fill_ring_empty_descs;
__u64 tx_ring_empty_descs;
};

When being copied to the userspace, 'stats' will not contain any uninitialized
'holes' between struct fields.

Fixes: 8aa5a33578e9 ("xsk: Add new statistics")
Suggested-by: Dan Carpenter <[e-mail]>
Signed-off-by: Peilin Ye <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Björn Töpel <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/xdp/xsk.c

···
 	switch (optname) {
 	case XDP_STATISTICS:
 	{
-		struct xdp_statistics stats;
+		struct xdp_statistics stats = {};
 		bool extra_stats = true;
 		size_t stats_size;