net: tun: fix misspellings using codespell tool

Some typos are found out by codespell tool:

$ codespell -w -i 3 ./drivers/net/tun.c
aovid  ==> avoid

Fix typos found by codespell.
Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/20210118111539.35886-1-dong.menglong@zte.com.cnSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/tun.c

@@ -2735,7 +2735,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
 		err = register_netdevice(tun->dev);
 		if (err < 0)
 			goto err_detach;
-		/* free_netdev() won't check refcnt, to aovid race
+		/* free_netdev() won't check refcnt, to avoid race
 		 * with dev_put() we need publish tun after registration.
 		 */
 		rcu_assign_pointer(tfile->tun, tun);