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: NMenglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/20210118111539.35886-1-dong.menglong@zte.com.cnSigned-off-by: NJakub Kicinski <kuba@kernel.org>