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.cn


Signed-off-by: Jakub Kicinski <kuba@kernel.org>