uuid: Add inline helpers to import / export UUIDs

Sometimes we may need to import UUID from or export to the raw buffer,
which is provided outside of kernel and can't be declared as UUID type.
With current API this operation will require an explicit casting
to one of UUID types and length, that is always a constant derived as sizeof
the certain UUID type.

Provide a helpful set of inline helpers to minimize developer's effort
in the cases when raw buffers are involved.

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