syscalls/fstatfs: Cleanup

* Delete some useless comments

* Make variables and functions static

Signed-off-by: Shuang Qiu <shuang.qiu@oracle.com>
Reviewed-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>