f2fs: check if file namelen exceeds max value

Dentry bitmap is not enough to detect incorrect dentries. So this patch
also checks the namelen value of a dentry.

Signed-off-by: Gong Chen <gongchen4@huawei.com>
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>