f2fs: fix to trigger fsck if dirent.name_len is zero

While traversing dirents in f2fs_fill_dentries(), if bitmap is valid,
filename length should not be zero, otherwise, directory structure
consistency could be corrupted, in this case, let's print related
info and set SBI_NEED_FSCK to trigger fsck for repairing.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>