afpd: Return the correct error for non-existent volume

Return kFPObjectNotFound rather than kFPParamErr when openvol() is
called with a non-existent volume.  This follows the specification and
Apple's implementation.

Signed-off-by: Ross Lagerwall <rosslagerwall@gmail.com>
Reviewed-by: Ralph Boehme <rb@sernet.de>