ovl: fix misspellings using codespell tool

A typo is found out by codespell tool:

$ codespell ./fs/overlayfs/
./fs/overlayfs/util.c:217: dependig  ==> depending

Fix a typo found by codespell.

Signed-off-by: Xiong Zhenwu <xiong.zhenwu@zte.com.cn>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>