net: disable preemption in dev_core_stats_XXX_inc() helpers

syzbot was kind enough to remind us that dev->{tx_dropped|rx_dropped}
could be increased in process context.

BUG: using smp_processor_id() in preemptible [00000000] code: syz-executor413/3593
caller is netdev_core_stats_alloc+0x98/0x110 net/core/dev.c:10298
CPU: 1 PID: 3593 Comm: syz-executor413 Not tainted 5.17.0-rc7-syzkaller-02426-g97aeb877de7f #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 check_preemption_disabled+0x16b/0x170 lib/smp_processor_id.c:49
 netdev_core_stats_alloc+0x98/0x110 net/core/dev.c:10298
 dev_core_stats include/linux/netdevice.h:3855 [inline]
 dev_core_stats_rx_dropped_inc include/linux/netdevice.h:3866 [inline]
 tun_get_user+0x3455/0x3ab0 drivers/net/tun.c:1800
 tun_chr_write_iter+0xe1/0x200 drivers/net/tun.c:2015
 call_write_iter include/linux/fs.h:2074 [inline]
 ...