ext4: fix a warning from sparse check for ext4_dir_llseek

ext4_dir_llseek is only used as a callback function, and no one calls
it directly. So make it as a static function in order to remove a
warning message from sparse check.

Signed-off-by: Zheng Liu <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/dir.c

···
  *
  * For non-htree, ext4_llseek already chooses the proper max offset.
  */
-loff_t ext4_dir_llseek(struct file *file, loff_t offset, int whence)
+static loff_t ext4_dir_llseek(struct file *file, loff_t offset, int whence)
 {
 	struct inode *inode = file->f_mapping->host;
 	int dx_dir = is_dx_dir(inode);