ceph: don't use special DIO path for encrypted inodes

Eventually I want to merge the synchronous and direct read codepaths,
possibly via new netfs infrastructure. For now, the direct path is not
crypto-enabled, so use the sync read/write paths instead.

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