ubifs: Fix ubifs_check_dir_empty() kernel-doc comment

Fix function name in fs/ubifs/dir.c kernel-doc comment
to remove warning found by running scripts/kernel-doc,
which is caused by using 'make W=1'.

fs/ubifs/dir.c:883: warning: expecting prototype for check_dir_empty().
Prototype was for ubifs_check_dir_empty() instead

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>