coda: Allow coda to be built when user namespace support is enabled

Now that the coda kernel to userspace has been modified to convert
between kuids and kgids and uids and gids, and all internal
coda structures have be modified to store uids and gids as
kuids and kgids it is safe to allow code to be built with
user namespace support enabled.

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