f2fs: fix a bug when using namehash to locate dentry bucket

In the following scenario,

1) we don't have the key and doing a lookup for encrypted file,
2) and the encrypted filename is big name

we should use fname->hash as name hash value instead of what is
calculated by fname->disk_name. Because in such case,
fname->disk_name is empty.

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