coda: Restrict coda messages to the initial pid namespace

Remove the slight chance that pids in coda messages will be
interpreted in the wrong pid namespace.

- Explicitly send all pids in coda messages in the initial pid
  namespace.
- Only allow mounts from processes in the initial pid namespace.
- Only allow processes in the initial pid namespace to open the coda
  character device to communicate with coda.

Cc: Jan Harkes <jaharkes@cs.cmu.edu>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>