uuid: Provide a GUID generator for raw buffer

In some cases we would like to generate a GUID and export it.  Though it
would require either casting to internal kernel types or an intermediate
buffer. Instead we may achieve this by supplying a pointer to raw buffer
and make a complimentary API to existing one for UUIDs.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David Sterba <dsterba@suse.com>