syzbot reported a data-race in virtnet_poll / virtnet_stats [1]

u64_stats_t infra has very nice accessors that must be used
to avoid potential load-store tearing.

[1]
BUG: KCSAN: data-race in virtnet_poll / virtnet_stats

read-write to 0xffff88810271b1a0 of 8 bytes by interrupt on cpu 0:
virtnet_receive drivers/net/virtio_net.c:2102 [inline]
virtnet_poll+0x6c8/0xb40 drivers/net/virtio_net.c:2148
__napi_poll+0x60/0x3b0 net/core/dev.c:6527
napi_poll net/core/dev.c:6594 [inline]
net_rx_action+0x32b/0x750 net/core/dev.c:6727
__do_softirq+0xc1/0x265 kernel/softirq.c:553
invoke_softirq kernel/softirq.c:427 [inline]
__irq_exit_rcu kernel/softirq.c:632 [inline]
irq_exit_rcu+0x3b/0x90 kernel/softirq.c:644
common_interrupt+0x7f/0x90 arch/x86/kernel/irq.c:247
asm_common_interrupt+0x26/0x40 arch/x86/include/asm/idtentry.h:636
__sanitizer_cov_trace_const_cmp8+0x0/0x80 kernel/kcov.c:306
jbd2_write_access_granted fs/jbd2/transaction.c:1174 [inline]
jbd2_journal_get_write_access+0x94/0x1c0 fs/jbd2/transaction.c:1239
__ext4_journal_get_write_access+0x154/0x3f0 fs/ext4/ext4_jbd2.c:241
ext4_reserve_inode_write+0x14e/0x200 fs/ext4/inode.c:5745
__ext4_mark_inode_dirty+0x8e/0x440 fs/ext4/inode.c:5919
ext4_evict_inode+0xaf0/0xdc0 fs/ext4/inode.c:299
evict+0x1aa/0x410 fs/inode.c:664
iput_final fs/inode.c:1775 [inline]
iput+0x42c/0x5b0 fs/inode.c:1801
do_unlinkat+0x2b9/0x4f0 fs/namei.c:4405
__do_sys_unlink fs/namei.c:4446 [inline]
__se_sys_unlink fs/namei.c:4444 [inline]
__x64_sys_unlink+0x30/0x40 fs/namei.c:4444
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff88810271b1a0 of 8 bytes by task 2814 on cpu 1:
virtnet_stats+0x1b3/0x340 drivers/net/virtio_net.c:2564
dev_get_stats+0x6d/0x860 net/core/dev.c:10511
rtnl_fill_stats+0x45/0x320 net/core/rtnetlink.c:1261
rtnl_fill_ifinfo+0xd0e/0x1120 net/core/rtnetlink.c:1867
rtnl_dump_ifinfo+0x7f9/0xc20 net/core/rtnetlink.c:2240
netlink_dump+0x390/0x720 net/netlink/af_netlink.c:2266
netlink_recvmsg+0x425/0x780 net/netlink/af_netlink.c:1992
sock_recvmsg_nosec net/socket.c:1027 [inline]
sock_recvmsg net/socket.c:1049 [inline]
____sys_recvmsg+0x156/0x310 net/socket.c:2760
___sys_recvmsg net/socket.c:2802 [inline]
__sys_recvmsg+0x1ea/0x270 net/socket.c:2832
__do_sys_recvmsg net/socket.c:2842 [inline]
__se_sys_recvmsg net/socket.c:2839 [inline]
__x64_sys_recvmsg+0x46/0x50 net/socket.c:2839
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x000000000045c334 -> 0x000000000045c376

Fixes: 3fa2a1df ("virtio-net: per cpu 64 bit stats (v2)")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>