ceph: add support to readdir for encrypted names

To make it simpler to decrypt names in a readdir reply (i.e. before
we have a dentry), add a new ceph_encode_encrypted_fname()-like helper
that takes a qstr pointer instead of a dentry pointer.

Once we've decrypted the names in a readdir reply, we no longer need the
crypttext, so overwrite them in ceph_mds_reply_dir_entry with the
unencrypted names. Then in both ceph_readdir_prepopulate() and
ceph_readdir() we will use the dencrypted name directly.

[ jlayton: convert some BUG_ONs into error returns ]

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>