uuid: Take const on input of uuid_is_null() and guid_is_null()

The null check functions do not and must not modify contents of the UUID
or GUID supplied.

Mark argument explicitly to reflect that.

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