uuid: discourage people from using UAPI header in new code

Discourage people from using UAPI header in new code by adding a note.

Link: https://lkml.kernel.org/r/20211216113552.81199-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>