uuid: hoist uuid_is_null() helper from libnvdimm

Hoist the libnvdimm helper as an inline helper to linux/uuid.h
using an auxiliary const variable uuid_null in lib/uuid.c.

[hch: also add the guid variant.  Both do the same but I'd like
to keep casts to a minimum]

The common helper uses the new abstract type uuid_t * instead of
u8 *.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
[hch: added guid_is_null]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

lib/uuid.c

@@ -21,6 +21,11 @@
 #include <linux/uuid.h>
 #include <linux/random.h>
 
+const guid_t guid_null;
+EXPORT_SYMBOL(guid_null);
+const uuid_t uuid_null;
+EXPORT_SYMBOL(uuid_null);
+
 const u8 guid_index[16] = {3,2,1,0,5,4,7,6,8,9,10,11,12,13,14,15};
 const u8 uuid_index[16] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};